You shudder at a split infinitive, know when to use 'that' or 'which' and would never confuse 'less' with 'fewer' – but are these rules always right, elegant or sensible, asks linguist Steven Pinker

Among the many challenges of writing is dealing with rules of correct usage: whether to worry about split infinitives, fused participles, and the meanings of words such as "fortuitous", "decimate" and "comprise". Supposedly a writer has to choose between two radically different approaches to these rules. Prescriptivists prescribe how language ought to be used. They uphold standards of excellence and a respect for the best of our civilisation, and are a bulwark against relativism, vulgar populism and the dumbing down of literate culture. Descriptivists describe how language actually is used. They believe that the rules of correct usage are nothing more than the secret handshake of the ruling class, designed to keep the masses in their place. Language is an organic product of human creativity, say the Descriptivists, and people should be allowed to write however they please.

It's a catchy dichotomy, but a false one. Anyone who has read an inept student paper, a bad Google translation, or an interview with George W Bush can appreciate that standards of usage are desirable in many arenas of communication. They can lubricate comprehension, reduce misunderstanding, provide a stable platform for the development of style and grace, and signal that a writer has exercised care in crafting a passage.

But this does not mean that every pet peeve, bit of grammatical folklore, or dimly remembered lesson from Miss Thistlebottom's classroom is worth keeping. Many prescriptive rules originated for screwball reasons, impede clear and graceful prose, and have been flouted by the best writers for centuries.

How can you distinguish the legitimate concerns of a careful writer from the folklore and superstitions? These are the questions to ask. Does the rule merely extend the logic of an intuitive grammatical phenomenon to more complicated cases, such as avoiding the agreement error in "The impact of the cuts have not been felt"? Do careful writers who inadvertently flout the rule agree, when the breach is pointed out, that something has gone wrong? Has the rule been respected by the best writers in the past? Is it respected by careful writers in the present? Is there a consensus among discerning writers that it conveys an interesting semantic distinction? And are violations of the rule obvious products of mishearing, careless reading, or a chintzy attempt to sound highfalutin?

A rule should be rejected, in contrast, if the answer to any of the following questions is "Yes." Is the rule based on some crackpot theory, such as that English should emulate Latin, or that the original meaning of a word is the only correct one? Is it instantly refuted by the facts of English, such as the decree that nouns may not be converted into verbs? Did it originate with the pet peeve of a self-anointed maven? Has it been routinely flouted by great writers? Is it based on a misdiagnosis of a legitimate problem, such as declaring that a construction that is sometimes ambiguous is always ungrammatical? Do attempts to fix a sentence so that it obeys the rule only make it clumsier and less clear?

Finally, does the putative rule confuse grammar with formality? Every writer commands a range of styles that are appropriate to different times and places. A formal style that is appropriate for the inscription on a genocide memorial will differ from a casual style that is appropriate for an email to a close friend. Using an informal style when a formal style is called for results in prose that seems breezy, chatty, casual, flippant. Using a formal style when an informal style is called for results in prose that seems stuffy, pompous, affected, haughty. Both kinds of mismatch are errors. Many prescriptive guides are oblivious to this distinction, and mistake informal style for incorrect grammar.

The easiest way to distinguish a legitimate rule of usage from a grandmother's tale is unbelievably simple: look it up. Consult a modern usage guide or a dictionary with usage notes. Many people, particularly sticklers, are under the impression that every usage myth ever loosed on the world by a self-proclaimed purist will be backed up by the major dictionaries and manuals. In fact, these reference works, with their careful attention to history, literature and actual usage, are the most adamant debunkers of grammatical nonsense. (This is less true of style sheets drawn up by newspapers and professional societies, and of manuals written by amateurs such as critics and journalists, which tend to mindlessly reproduce the folklore of previous guides.)

What follow are 10 common issues of grammar selected from those that repeatedly turn up in style guides, pet-peeve lists, newspaper language columns and irate letters to the editor.

and, because, but, or, so, also

Many children are taught that it is ungrammatical to begin a sentence with a conjunction. That's because teachers need a simple way to teach them how to break sentences, so they tell them that sentences beginning with "and" and other conjunctions are ungrammatical. Whatever the pedagogical merits may be of feeding children misinformation, it is inappropriate for adults. There is nothing wrong with beginning a sentence with a conjunction. "And", "but" and "so" are indispensable in linking individual sentences into a coherent passage, and they may be used to begin a sentence whenever the clauses being connected are too long or complicated to fit comfortably into a single megasentence. The conjunction "because" can also happily sit at the beginning of a sentence. Most commonly it ends up there when it introduces an explanation that has been preposed in front of a main clause, as in: "Because you're mine, I walk the line." But it can also kick off a single clause when the clause serves as the answer to a why question: "'Why can't I have a pony?' 'Because I said so.'"

dangling modifiers

Do you see a problem with the sentences that follow?

"Checking into the hotel, it was nice to see a few of my old classmates in the lobby."

"Turning the corner, the view was quite different."

"In order to contain the epidemic, the area was sealed off."

According to an old rule about "dangling modifiers", these sentences are ungrammatical. The rule decrees that the implied subject of the modifier (the one doing the checking, turning, and so on) must be identical to the overt subject of the main clause (it, the view, and so on). Most copy editors would recast the main clause, supplying it with a subject to which the modifier can be properly fastened:

"Checking into the hotel, I was pleased to see a few of my old classmates in the lobby."

"Turning the corner, I saw that the view was quite different."

"In order to contain the epidemic, authorities sealed off the area."

Newspaper columns on usage are filled with apologies for "errors" like these. Danglers are extremely common, not just in deadline-pressured journalism but in the works of distinguished authors. Considering how often these forms turn up in edited prose and how readily they are accepted even by careful readers, two conclusions are possible: either dangling modifiers are a particularly insidious grammatical error for which writers must develop sensitive radar, or they are not grammatical errors at all. (Did you notice the dangler in the sentence before last?)

The second conclusion is the right one: some dangling modifiers should be avoided, but they are not grammatical errors. The problem with dangling modifiers is that their subjects are inherently ambiguous and sometimes a sentence will inadvertently attract a reader to the wrong choice, as in "When a small boy, a girl is of little interest."

But some so-called danglers are perfectly acceptable. Many participles have turned into prepositions, such as "according", "allowing", "concerning", "considering", "excepting", "following", "given", "granted", "owing", "regarding" and "respecting", and they don't need subjects at all. Inserting "we find" or "we see" into the main clause to avoid a dangler can make the sentence stuffy and self-conscious. More generally, a modifier can dangle when its implied subject is the writer and the reader. The decision of whether to recast a sentence to align its subject with the subject of a modifier is a matter of judgment, not grammar. A thoughtlessly placed dangler can confuse the reader or slow them down, and occasionally it can lure them into a ludicrous interpretation. Also, even if a dangler is in no danger of being misinterpreted, enough readers have trained themselves to spot danglers that a writer who leaves it incurs the risk of being judged as slovenly. So in formal styles it's not a bad idea to keep an eye open for them and to correct the obtrusive ones.

like, as, such as

Photograph: Apic/Getty Images

Long ago, in the Mad Men era when cigarettes were advertised on radio and television, every brand had a slogan. "I'd walk a mile for a Camel." "Lucky Strike means fine tobacco." "Come to where the flavor is. Come to Marlboro Country." And most infamously, "Winston tastes good, like a cigarette should."

The infamy did not come from the fact that the company was using a catchy jingle to get people addicted to carcinogens. It came from the fact that the jingle allegedly contained a grammatical error. "Like" is a preposition, said the accusers, and may take only a noun phrase object, as in "crazy like a fox" or "like a bat out of hell". It is not a conjunction and so may not be followed by a clause. The New Yorker sneered at the error, Ogden Nash wrote a poem about it, Walter Cronkite refused to say it on the air, and style guide icons Strunk and White declared it illiterate. The slogan, they all agreed, should have been "Winston tastes good, as a cigarette should." The advertising agency and the tobacco company were delighted by the unpaid publicity and were only too happy to confess to the error in the coda, "What do you want, good grammar or good taste?"

Like many usage controversies, the brouhaha over "like a cigarette should" is a product of grammatical ineptitude and historical ignorance. The ad's use of "like" with a clause was not a recent corruption; the combination has been in use for 600 years. It has been used in literary works by dozens of great writers (including William Shakespeare, Charles Dickens, Mark Twain, HG Wells and William Faulkner) and has flown beneath the radar of the purists themselves, who have inadvertently used it in their own style guides. This does not show that purists are only human and sometimes make errors; it shows that the alleged error is not an error. The RJ Reynolds Tobacco Company was confessing to the wrong crime; its slogan was perfectly grammatical. Writers are free to use either "like" or "as", mindful only that "as" is a bit more formal, and that the Winston-tastes-good controversy became such a bloody shirt in the grammar wars that readers may mistakenly think the writer has made an error.

A related superstition, ruthlessly enforced by many copy editors, is that "like" may not be used to introduce examples, as in "Many technical terms have become familiar to laypeople, like 'cloning' and 'DNA'." They would correct it to "such as 'cloning' and 'DNA'". According to this guideline, "like" may be used only for resemblance to an exemplar, as in "I'll find someone like you" and "Poems are made by fools like me." Few writers consistently follow this bogus rule. "Such as" is more formal than "like", but both are legitimate.

preposition at the end of a sentence

Winston Churchill did not, as legend has it, reply to an editor who had corrected his prose with "This is pedantry up with which I will not put." Nor is that witticism (originally from a 1942 Wall Street Journal article) a particularly good example of the construction that linguists call "preposition stranding", as in "Who did you talk to?" or "That's the bridge I walked across." The particle "up" is an intransitive preposition and does not require an object, so even the most pedantic of pedants would have no objection to a phrase like "This is pedantry with which I will not put up."

Though the attribution and the example are spurious, the mockery is appropriate. The prohibition against clause-final prepositions is considered a superstition even by the language mavens, and it persists only among know-it-alls who have never opened a dictionary or style manual to check. There is nothing, repeat nothing, wrong with "Who are you looking at?" or "The better to see you with" or "We are such stuff as dreams are made on" or "It's you she's thinking of". The pseudo-rule was invented by John Dryden based on a silly analogy with Latin (where the equivalent to a preposition is attached to the noun and cannot be separated from it) in an effort to show that Ben Jonson was an inferior poet. As the linguist Mark Liberman remarked, "It's a shame that Jonson had been dead for 35 years at the time, since he would otherwise have challenged Dryden to a duel, and saved subsequent generations a lot of grief."

The alternative to stranding a preposition at the end of a clause is allowing it to accompany a "wh" word to the front, a rule that the linguist JR (Haj) Ross dubbed pied-piping, because it reminded him of the way that the Pied Piper lured the rats out of the village of Hamelin. The standard question rule in English converts "You are seeing what?" into "What are you seeing?" and hence "You are looking at what?" into "What are you looking at?" The pied-piping rule allows the "what" to pull the "at" with it to the front of the sentence, yielding "At what are you looking?" and similar clauses, such as "The better with which to see you," or "It's you of whom she's thinking."

How should you choose? Most obviously, pied-piping sounds better in a formal style. Abraham Lincoln knew what he was doing at the graves of the fallen soldiers at Gettysburg when he vowed "increased devotion to that cause for which they gave the last full measure of devotion", rather than "increased devotion to that cause which they gave the last full measure of devotion for". The problem with stranding a preposition is that it can end the sentence with a word that is too lightweight to serve as its focal point, making the sentence sound like "the last sputter of an engine going dead". By the same principle, a preposition should be stranded at the end of a sentence when it contributes a crucial bit of information, as in "music to read by", "something to guard against", or when it pins down the meaning of an idiom, as in "It's nothing to sneeze at" or "He doesn't know what he's talking about".

predicative nominative

When you come home after a day at the office, do you call out, "Hi, honey, it's I"? If you do, you are the victim of a schoolteacher rule that insists that a pronoun serving as the complement of "be" must be in nominative case (I, he, she, we, they) rather than accusative case (me, him, her, us, them). According to this rule, Psalms (120:5), Isaiah (6:5), Jeremiah (4:31), and Ophelia should have cried out, "Woe is I," and the cartoon possum Pogo should have reworded his famous declaration as "We have met the enemy, and he is we."

The rule is a product of the usual three confusions: English with Latin, informal style with incorrect grammar and syntax with semantics. Accusative predicates have been used for centuries by many respected writers (including Samuel Pepys, Ernest Hemingway and Virginia Woolf), and the choice between "It is he" and "It is him" is strictly one of formal versus informal style.

split infinitives

Most mythical usage rules are merely harmless. The prohibition of split infinitives (as in "Are you sure you want to permanently delete all the items and subfolders in the 'Deleted Items' folder?") and the even more sweeping prohibition of "split verbs" (as in "I will always love you" and "I would never have guessed") is downright pernicious. During the 2009 presidential inauguration, Chief Justice John Roberts, a famous stickler for grammar, could not bring himself to have Barack Obama "solemnly swear that I will faithfully execute the office of president of the United States". Abandoning his strict constructionism, Roberts unilaterally amended the Constitution and had Obama "solemnly swear that I will execute the office of president to the United States faithfully." The garbled oath raised fears about whether the transfer of power had been legitimate, and so they repeated the oath verbatim, split verb and all, in a private meeting later that afternoon.

The very terms "split infinitive" and "split verb" are based on a thick-witted analogy to Latin, in which it is impossible to split a verb because it consists of a single word, such as amare, "to love". But in English, the so-called infinitive "to write" consists of two words, not one: the subordinator "to" and the plain form of the verb "write", which can also appear without "to" in constructions such as "She helped him pack" and "You must be brave." There is not the slightest reason to interdict an adverb from the position before the main verb, and great writers in English have placed it there for centuries. Indeed, the spot in front of the main verb is often the most natural resting place for an adverb, and sometimes it is the only resting place. Unsplitting the infinitive in the New Yorker cartoon caption "I'm moving to France to not get fat" (yielding "I'm moving to France not to get fat") would garble the meaning, and doing so with "Profits are expected to more than double this year," would result in gibberish: "Profits are expected more than to double this year."

More generally, the preverbal position is the only one in which the adverb unambiguously modifies the verb. In a sentence in which the author may have taken pains to unsplit an infinitive, such as "The board voted immediately to approve the casino", the reader has to wonder whether it was the vote that was immediate, or the approval. With the infinitive left unsplit – "The board voted to immediately approve the casino" – it can only be the approval. This does not mean that infinitives should always be split. Indeed, it's a good habit to at least consider moving an adverb to the end of the verb phrase. If the adverb conveys important information, it belongs there; if it doesn't (such as "really", "just", "actually" and other hedges), it might be a verbal fluffball that is best omitted altogether. And since there are benighted sticklers out there who will mistakenly accuse you of making an error when you split an infinitive, you might as well not ask for trouble if it makes no difference to the sentence anyway.

Finally, in many cases a quantifier naturally floats leftward away from the verb, unsplitting the infinitive:

"I find it hard to specify when to not split an infinitive."

"I find it hard to specify when not to split an infinitive."

The unsplit versions sound more elegant to me, though I can't be sure that my ears haven't been contaminated by a habit of cravenly unsplitting infinitives to avoid spitballs from the Gotcha! Gang.

that and which

Many spurious rules start out as helpful hints intended to rescue indecisive writers from paralysis when faced with a choice provided by the richness of English. These guides for the perplexed also make the lives of copy editors easier, so they may get incorporated into style sheets. Before you know it, a rule of thumb morphs into a rule of grammar, and a perfectly innocuous (albeit second-choice) construction is demonised as incorrect. Nowhere is this transition better documented than with the phony but ubiquitous rule on when to use "which" and when to use "that".

According to the traditional rule, the choice depends on which of two kinds of relative clause the word is introducing. A nonrestrictive relative clause is set off by commas, dashes or parentheses, as in "The pair of shoes, which cost five thousand dollars, was hideous." A restrictive relative clause is essential to the meaning of the sentence, often because it pinpoints the referent of the noun from among a set of alternatives. If we were narrating a documentary about Imelda Marcos's vast shoe collection and wanted to single out one of the pairs by how much she paid for it and then say something about that pair alone, we would write "The pair of shoes that cost £5,000 was hideous." The choice between "that" and "which", according to the rule, is simple: nonrestrictive relative clauses take "which"; restrictive relative clauses take "that".

One part of the rule is correct: it's odd to use "that" with a nonrestrictive relative clause, as in "The pair of shoes, that cost £5,000, was hideous." So odd, in fact, that few people write that way, rule or no rule.

The other part of the rule is utterly incorrect. There is nothing wrong with using "which" to introduce a restrictive relative clause, as in "The pair of shoes which cost £5,000 was hideous." Indeed, with some restrictive relatives, "which" is the only option, such as "That which doesn't kill you makes you stronger" and "The book in which I scribbled my notes is worthless." Even when "which" isn't mandatory, great writers have been using it for centuries, as in the King James Bible's "Render therefore unto Caesar the things which are Caesar's" and Franklin Roosevelt's "a day which will live in infamy".

So what's a writer to do? The real decision is not whether to use "that" or "which" but whether to use a restrictive or a nonrestrictive relative clause. If a phrase that expresses a comment about a noun can be omitted without substantially changing the meaning, and if it would be pronounced after a slight pause and with its own intonation contour, then be sure to set it off with commas (or dashes or parentheses): "The Cambridge restaurant, which had failed to clean its grease trap, was infested with roaches." Having done so, you don't have to worry about whether to use "that" or "which", because if you're tempted to use "that" it means either that you are more than 200 years old or that your ear for the English language is so mistuned that the choice of "that" and "which" is the least of your worries.

If, on the other hand, a phrase provides information about a noun that is crucial to the point of the sentence (as in "Every Cambridge restaurant which failed to clean its grease trap was infested with roaches", where omitting the italicised phrase would radically alter the meaning), and if it is pronounced within the same intonation contour as the noun, then don't set it off with punctuation. As for the choice you now face between "which" and "that": if you hate making decisions, you won't go wrong if you use "that".

who and whom

When Groucho Marx was once asked a long and orotund question, he replied, "Whom knows?" A 1928 short story by George Ade contains the line "'Whom are you?' he said, for he had been to night school." In 2000 the comic strip Mother Goose and Grimm showed an owl in a tree calling "Whom" and a raccoon on the ground replying "Show-off!" A cartoon entitled "Grammar Dalek" shows one of the robots shouting, "I think you mean Doctor Whom!"

The popularity of "whom" humour tells us two things about the distinction between "who" and "whom". First, "whom" has long been perceived as formal verging on pompous. Second, the rules for its proper use are obscure to many speakers, tempting them to drop "whom" into their speech whenever they want to sound posh.

So you may be inclined to agree with the writer Calvin Trillin when he wrote, "As far as I'm concerned, 'whom' is a word that was invented to make everyone sound like a butler." But this is an overstatement. There are times when even non-butlers need to know their "who" from their "whom".

It ought to be straightforward. The distinction between "who" and "whom" is identical to that between "he" and "him" or "she" and "her", which no one finds difficult. We say "He kissed the bride," so we ask "Who kissed the bride?" We say "Henry kissed her," so we ask "Whom did Henry kiss?" But even after a century of nagging by prescriptive grammarians, the "who–whom" distinction remains tenuous in speech and informal writing. Only the stuffiest prig would say "Whom are you going to believe, me or your own eyes?" "It's not what you know; it's whom you know," or "Do you know whom you're talking to?" And when people do try to write with "whom", they often get it wrong, as in "Whomever installed the shutters originally did not consider proper build out."

Like the subjunctive mood, the pronoun "whom" is widely thought to be circling the drain. Indeed, tabulations of its frequency in printed text confirm that it has been sinking for almost two centuries. The declining fortunes of "whom" may represent not a grammatical change in English but a cultural change in Anglophones, namely the informalisation of writing, which makes it increasingly resemble speech. But it's always risky to extrapolate a downward slope all the way to zero, and since the 1980s the curve seems to be levelling off. Though "whom" is pompous in short questions and relative clauses, it is a natural choice in certain other circumstances, even in informal speech and writing. We still use "whom" in double questions like "Who's dating whom?", and in fixed expressions like "To whom it may concern" and "With whom do you wish to speak?". A scan of my email turns up hundreds of hits for "whom" in unmistakably informal sentences such as "Not sure if you remember me; I'm the fellow from Casasanto's lab with whom you had a hair showdown while at Hunter College."

The best advice to writers is to calibrate their use of "whom" to the complexity of the construction and the degree of formality they desire. If William Safire, who wrote the New York Times' "On Language" column and coined the term "language maven" in reference to himself, could write, "Let tomorrow's people decide who they want to be president," so can you.

very unique

They say you can't be a little bit married or a little bit pregnant, and purists believe that the same is true for certain other adjectives. One of the commonest insults to the sensibility of the purist is the expression "very unique" and other phrases in which an "absolute" or "incomparable" adjective is modified by an adverb of degree such as "more", "less", "somewhat", "quite" or "almost". Uniqueness, the purists say, is like marriage and pregnancy: something is either unique (one of a kind) or not unique, so referring to degrees of uniqueness is meaningless. Nor can one sensibly modify "absolute", "certain", "complete", "equal", "eternal", "perfect" or "the same". One may not write, for instance, that one statement is "more certain" than another, or that an apartment is "relatively perfect".

A glance at the facts of usage immediately sets off Klaxon horns. Great writers have been modifying absolute adjectives for centuries, including the framers of the American Constitution, who sought "a more perfect union". Many of the examples pass unnoticed by careful writers, including "nothing could be more certain" and "there could be no more perfect spot". Though the phrase "very unique" is universally despised, other modifications of "unique" are unobjectionable, as when Martin Luther King wrote, "I am in the rather unique position of being the son, the grandson, and the great grandson of preachers."

Here is the flaw in the purists' logic. Uniqueness is not like pregnancy and marriage; it must be defined relative to some scale of measurement. I am told that all snowflakes are unique, and so they may be under a microscope, but frankly, they all look the same to me. Conversely, each of the proverbial two peas in a pod is unique if you squint hard enough through a magnifying glass. Does this mean that nothing is unique, or does it mean that everything is unique? The answer is neither: the concept "unique" is meaningful only after you specify which qualities are of interest to you and which degree of resolution or grain size you're applying. Calling something "quite unique" or "very unique" implies that the item differs from the others in an unusual number of qualities, that it differs from them to an unusual degree, or both. In other words, pick any scale or cutoff you want, and the item will still be unique.

This doesn't mean that you should go ahead and use "very unique". "Very" is a soggy modifier in the best of circumstances, and the combination with "unique" grates on enough readers that it's wise to avoid it.

count nouns, mass nouns and "ten items or less"

English speakers can conceptualise aggregates as discrete things, which are expressed as plural count nouns, such as "pebbles" or as continuous substances, which are expressed as mass nouns, such as "gravel". Some quantifiers are choosy as to which they apply to. We can talk about "many pebbles" but not "much pebbles", "much gravel" but not "many gravel". Some quantifiers are not choosy: we can talk about "more pebbles" or "more gravel".

Photograph: Adam Gale

Now, you might think that if "more" can be used with both count and mass nouns, so can "less". But it doesn't work that way: you may have "less gravel", but most writers agree that you can only have "fewer pebbles", not "less pebbles". This is a reasonable distinction, but purists have extended it with a vengeance. The sign over supermarket express checkout lanes, "Ten Items or Less", is a grammatical error, they say, and as a result of their carping upscale supermarkets have replaced the signs with "Ten Items or Fewer". By this logic, off licences should refuse to sell beer to customers who are "fewer than 21 years old" and law-abiding motorists should drive at "fewer than 70 miles an hour". And once you master this distinction, well, that's one fewer thing for you to worry about.

Clearly, the purists have botched the "less-fewer" distinction. "Less" is perfectly natural with a singular count noun, as in "one less car" and "one less thing to worry about". It's also natural when the entity being quantified is a continuous extent and the count noun refers to units of measurement, such as "21 years old" and "70 miles an hour"; like the 1-11 scale on Nigel Tufnel's favourite amplifier in This Is Spinal Tap, the units are arbitrary. And "less" is idiomatic in certain expressions in which a quantity is being compared to a standard, such as "Describe yourself in 50 words or less." Like many dubious rules of usage, the less-fewer distinction has a smidgen of validity as a pointer of style. In cases where "less" and "fewer" are both available, such as "Less/fewer than 20 of the students voted", "fewer" is the better choice because it enhances vividness and concreteness. But that does not mean that "less" is a grammatical error.

• Steven Pinker's The Sense of Style: the Thinking Person's Guide to Writing in the 21st Century is published next month (Allen Lane, £16.99). To order it for £13.59 with free UK p&p call Guardian book service on 0330 333 6846 or go to guardianbookshop.co.uk.

• This article was amended on 19 August 2014. An earlier version referred to Shakespeare's, rather than the King James Bible's, "Render therefore unto Caesar the things which are Caesar's".