"I'M TALL, but my brother is taller than __" How do you complete this sentence? There are at least three ways. But only one is uncontroversial. The ways in which the other two are irksome illuminate a good bit about English grammar. The completely correct way to finish the sentence is "than I am." But many times, English-speakers want to truncate what comes after "than". What goes in that slot? Many people think the only correct answer is I: after all, you are shortening I am. But my brother is taller than I feels stilted and stuffy to many perfectly educated native speakers, who would prefer my brother is taller than me. What's going on?

The question is whether than is a conjunction or a preposition—and the answer is that it is both. Conjunctions join clauses (a clause has a verb and usually a subject as well): and, but and or are the most common conjunctions, but than is also a conjunction when it joins clause like he loves flowers more than she does. Prepositions, in contrast, take a simpler object, a noun or noun phrase. And when that noun becomes a pronoun, it is in what is usually called the objective case. Me, him, her and them, not I, he, she and they. And so than acts like a preposition in taller than me.

Some believe that than can only be a conjunction. But this is plainly not how most English-speakers speak or write. Taller than me is not some recent grammatical laziness; it has deep and sturdy roots in the finest English. Prepositional than appears in the 1560 "Geneva Bible" translation ("a fool's wrath is heavier than them both"), Shakespeare ("a man no mightier than thyself or me"), Swift ("she suffers hourly more than me"), Samuel Johnson ("No man had ever more discernment than him") and so on. Taller than I is all right as well, and is more common in writing, but has a considerably more formal feel. So choose the one that works with the style you're aiming for. And if someone tells you not to, cite Shakespeare, Swift and Johnson and ask the self-appointed scold if he thinks he knows better than them.