In grammar, the general rule is that you shouldn’t use two negatives. It’s incorrect to say I didn’t see no one, although people will know what you mean. The correct thing is, obviously, I didn’t see anyone.
Contractual drafters like to avoid double negatives because they give rise to ambiguity. The concern isn’t so much over a sentence like I didn’t see know no one, but with repeated use of not or other negatives in a long, complicated sentence where the net effect may be uncertain.
Not all languages are averse to the double negative. Think of French: ne … pas, ne … jamais, ne … rien. And in Greek, negation is piled on negation to create a multi-layered expression of dissent, refusal or denial (something the Greeks have been doing a lot of in recent years).
There are times in English when a double negative can express a subtle shading of doubt or negativity, without outright contradiction. To say I don’t disagree is a bit less certain than the categorical I agree, and that may be a desirable effect. I’ve always wanted to throw this one into conversation, just to see the reaction: I’m not sure that I don’t disagree with you. How’s that for hedging?
Not enough negativity
Sometimes people underuse the negative.
A gaffe that grammarians love to point out is I could care less, when it’s used to express lack of interest in something. If you think about it, it actually means you do care about whatever it is, because it would be possible to care to a lesser degree. You should say you couldn’t care less if you don’t care at all.
A word or two about nor would be in order too. The correct construction is neither … nor (and either … or). By way of example: It is neither fish nor flesh, nor good red herring; either you understand this or you don’t.
As a general rule, don’t use nor if your previous negative is just not (or no). This is wrong: The Tenant agrees that it will not use nor permit the use of any asbestos, polychlorinated biphenyls … So is this (from Lexology): Discontinuing Free App is Not Unfair Competition Nor False Advertising under California Law. When more words pile up between not and what should be or, it’s easy to lose the plot and put a misplaced nor.
Exception: use nor to reinforce a previous negative statement, where nor is preceded by some punctuation (comma, semi-colon, colon, dash) and introduces a new clause. Example: I was not present at the scene of the crime, nor was I even aware that my friends were there.
In short, think about whether – and how – to negate. And remember the words of that immortal bard, Sir Tom Jones: It’s not unusual …
Next time: singular or plural?