Comparing "bu" and "mei"

不 denies a habit, a state or an intention; 没 denies that something happened.

不 + habit / state / future · 没 + didn't happen

How it works

我今天晚上不吃饭 — I'm not eating tonight (a decision about the future). 昨天晚上我没吃饭 — I didn't eat last night (it didn't happen).

So the split is aspect, not tense: 不 for what you don't do or won't do, 没 for what did not occur.

Two fixed rules make it easier. 有 always takes 没. And 没 never combines with 了.