I don’t know nothing about conditional sentences.
(This article has been slightly updated from the book)
If there was one typical student mistake that would be similar to the act of using a piece of chalk on a blackboard that produces a sound that sets your teeth on edge (an idiomatic phrase that means, in this context, a noise that sounds so irritating that you get the sensation of not only feeling it on your teeth, but also throughout your body), then this is probably it - well, it is a problem to those who like to listen to standard English and not everyday spoken English, either colloquial or in a particular dialect.
Double negation (or to give its more popular term, double negative) is the use of two negative words that occur in the same part of the sentence, or clause. It is often used in everyday conversation to express a negative idea: I haven’t done nothing. I didn’t see nobody. We don’t need no education, we don’t need no thought control (thank you Pink Floyd for that one). I can’t get no satisfaction (The Rolling Stones).
There are many people who believe that in English grammar, just like in science, two negatives will usually cancel each other out. This results in a positive statement, so the above examples should be interpreted as I’ve done something, I saw somebody, I need education. I need thought control. I can get satisfaction. So the example above is literally saying: I know something about conditional sentences. (I think you meant to say I know nothing about conditional sentences.)
Now oxforddictionaries.com do point out that double negatives are used in other languages (and were used in English until around the 16th century), and as already mentioned, are used by many native speakers either as colloquialisms or even part of their dialects. The double negative is being used to further intensify something that is already being expressed in a negative context. But as far as standard English is concerned, particularly when written in a formal context, single negatives should be used: I haven’t done anything. I didn’t see anybody. We don’t need education. We don’t need thought control. I can’t get satisfaction. I don’t know anything about conditional sentences. But if you decide to use a double negative in an informal conversation, most native speakers would be happy with it as you are simply emphasizing a point.
But there is always an exception somewhere. (Really?)
Indeed, it is clear in these examples that the two negatives make a positive meaning, and in this form is generally considered to be standard English: Winning the lottery? It would make me happy. It can't not make me happy! You know, I never do not drink. I'm quite happy to pour myself a glass of whiskey and just continue.
Here are some more examples of two negatives that lead to something positive. Look at this example: She was not unhappy with the result. Using not with unhappy suggests that she still has some problems with the result, but this new situation is still better than the original. If you compare this with she was unhappy with the result, then this tells us that for sure, she was 100 per cent unhappy.
Indeed, many negative words formed with prefixes can form clauses in this way: The board weren’t in complete disagreement (the board members disagreed with many things, but not everything); It wasn’t exactly illegal (but it was not completely legal either). Well, the advice wasn’t unhelpful (there were some helpful points, but it could have been more helpful).