Negatives - double negatives
Many writers have been told at some stage in their educational careers to "avoid the double negative."
This is good advice for academic writing. Many languages make a negative stronger by repeating it. Some languages, notably French, habitually use two words to make one negative ("Je ne suis pas français", for example. Here there are two negative words, translating one English negative: "I am not French").
The rules of formal English are more like those governing a minus sign in mathematics. "Two minuses make a plus", we were taught at school. In formal English, as in Maths, two negative words cancel each other out: they make a positive. The guilty schoolchild may say "I never done nothing." Although it is perfectly obvious what is meant, a pedantic schoolteacher may point out that it is, in the strict interpretation of the English language, a confession. The child is saying (or so the teacher pretends to understand) that there was no moment in time ('never') at which he was not committing a misdeed ('nothing'). Therefore, the pedant reasons, he must have been doing bad things all the time.
This is an area where precise formal English differs clearly from normal colloquial everyday English. Over time, the rules have changed. Shakespeare and Chaucer both used double negatives - see Negatives - double negatives in older English literature.