Q:

Je ne [verb] pas (de/le/les) after negative?

I have the sentence: Je n'aimais pas (les/des/de) légumes et je refusais d'en manger.
I was taught in French class to use 'de' after a negative always.
I asked one of my friends who was French, he corrected it to 'les'. Now I'm confused... Please help :)

A:

I was taught in French class to use 'de' after a negative always.

You were taught wrong or you learned the rule wrong. The rule is: replace an indefinite article, such as un, une or des, with de in a negative sentence. (There are some possible exceptions to this, as described here, but it's generally a good guide.) A definite article, such as le, la or les, remains the same in a negative sentence. You cannot replace it with de; that would change the meaning. 
Your friend suggested using les here simply because in French the definite article is used to refer to things in general. "Vegetables" in English translates to "les  légumes" in French. See the answers to the following question for more information about using the definite article after the verb aimer: La difference entre « j'aime le fromage » et « j'aime du fromage » (The difference is that the first is correct, and the second isn't)

