http://www.eslprintables.com/forum/topic.asp?id=49332&page=2

We didn �t see nothing. [ = We saw nothing.]

http://www.oxforddictionaries.com/words/double-negatives

The rules dictate that the two negative elements cancel each other out to give a positive statement instead, so that the sentence ‘I don’t know nothing’ could literally be interpreted as ‘I do know something’.

Double negatives are standard in many other languages and they were also a normal part of English usage until some time after the 16^th century. They’re still widely used in English dialectswhere they don’t seem to cause any confusion as to the intended meaning. Nevertheless, they aren’t considered acceptable in current standard English and you should avoid them in all but very informal situations. Just use a single negative instead:

We didn’t see anything.

She never danced with anyone.

There is one type of double negative that is considered grammatically correct and which people use to make a statement more subtle. Take a look at the following sentence:

I am not unconvinced by his argument.

The use of not together with unconvinced suggests that the speaker has a few mental reservations about the argument. The double negative creates a nuance of meaning that would not be present had the speaker just said:

I am convinced by his argument.

A double negative occurs when you use two negative words or constructions within a single clause. In some languages, such as Russian, Macedonian, Serbian and Croation, if you want to express a negative connotation, you need to use a negative throughout the phrase or sentence. In English, this is not the case. When you have a single clause - one cohesive idea with a subject and an action - you should only have one negative within that clause.

• For example, it would be wrong to say “We don’t need no education" (and perhaps Pink Floyd was wrong and they did in fact need an education to understand why double negative usage was wrong!)
• However, it would not be wrong to say "I don’t need an education because I don’t believe education is important." Even though don’t is a negative used twice in the sentence, "I don’t need an education" was one clause and "I don’t believe education is important" is a separate clause entirely.