FEW grammatical issues in history can have been quite as consequential. In Helsinki, Donald Trump rhetorically sized up the statements of his own director of national intelligence against those of Vladimir Putin, the former KGB spy standing a few feet away. Did Russia interfere with the election of 2016? “My people came to me. They said they think it’s Russia. I have President Putin. He just said it’s not Russia. I will say this: I don’t see any reason why it would be.”

Then yesterday, Mr Trump issued what is, for him, a unicorn-feather of a statement, the rarest of things: a retraction. “The sentence should have been ‘I don’t see any reason why it wouldn’t be Russia.’ Sort of a double negative.”

But was it a mistake? Could the president have plausibly said the exact opposite of what he meant at such a critical moment? Most commentary has focused on the wider context: the other things Mr Trump said during the press conference, and whether his correction is consonant with them. But a little close reading of the sentence in question itself may be helpful too.

People really do say the opposite of what they mean all the time, a type of error linguists call “misnegation”. Mark Liberman, a linguist at the University of Pennsylvania who runs the Language Log blog, has found dozens of such instances over the years, even in writing. People make far more errors in speech. Misnegation happens because it can be hard to keep track of the number of negations in a sentence. Some negation words are obvious: not is the clearest one in English. But there are many other negators: take hardly. It seems like an adverb, but in many contexts it is actually a negator: “He is hardly a brilliant negotiator” means he is not a brilliant negotiator.

As a result many words—some of them obvious, some of them less so—can flip the meaning of a sentence. And in longer, more complicated sentences, it is easy to lose track of how many someone has used. As Mr Liberman noted, General Michael Hayden, who ran the CIA under George W. Bush, said recently:

“I would not be surprised if this were not the last indictment we see that doesn’t mention an American.”

General Hayden said the opposite of what he meant, if you carefully count up the negations. Even three can be enough to lose track of, if the sentence is otherwise slightly complicated; the brain is doing several things at once when speaking, and tracking negations seems to place a heavy load on working memory. So do multiple embedded clauses. In General Hayden’s case, there is one main clause and three subordinate ones (the negators are indicated in bold).

I would not be surprised

If this were not the last indictment

we see

that doesn’t mention an American.

A third factor, Mr Liberman has found, is “irrealis” clauses, those that mention a hypothetical or currently untrue statement. Misnegation often happens in irrealis situations and, here, that “if this were not the last indictment…” clause is just that. In other words, this is a canonical misnegation. Had Mr Hayden tried a simpler option—“I wouldn’t be surprised if future indictments mention an American,” two clauses, one negation, no irrealis—he surely would have nailed it. What General Hayden meant was obvious: he thought that forthcoming indictments would definitely include Americans.

Is it plausible that Mr Trump misnegated in Helsinki? The eagle-eyed will note just one negative word in his statement: don’t. Mr Trump claims a second word should have been negative, too: wouldn’t. The differences between General Hayden’s sentence and Mr Trump’s are clear. This is a relatively flat sentence. In nine words, it has just one subordinate clause: “why it would be”. With its four clauses and three negations (where there should have been two), General Hayden’s sentence is not twice as hard but exponentially harder to process than Mr Trump’s. In addition, there are no irrealis clauses in Mr Trump’s. In other words, this is a pretty straightforward bit of grammar, not the kind that people commonly screw up.

What about the wider context? Mr Trump said “I will say this: I don’t see any reason why it would be.” “I will say this” is kind of a pre-announcement. Strictly, nobody needs to say this ever. People are always about to say the thing they are about to say. But it does serve a function: in effect, “Listen up. I’m going to give you a short pause to gather your attention, because this next sentence is particularly important.” It also gives the speaker a moment to plan. This was not a throwaway sentence where Mr Trump’s own attention might have wandered. He announced its importance.

In other words, it’s possible—but highly unlikely—that Mr Trump really misspoke. Add in the painful body language and laboured reading of a prepared statement announcing that he had, and it is hard to escape the conclusion that someone convinced him to eat his words from Helsinki upon his return home. The damage done remains.