Bu and mei(you)
Bu and mei(you) can both be used to negate verbs, however they are used differently. Let's first look at how they are used for statements in the past and present.
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Past and present statements
Perhaps the most intuitive explanation of the difference between 不 and 没（有）is that 不 negates actions in the present whereas 没 negates actions in the past. For example:
- 我 不 去 (I'm not going)
- 他 没 去 (I didn't go)
- 我 没 告诉 他！ (Honestly, I didn't tell him)
- 我 不 告诉 你 (I'm not telling you)
At first this is a good way to remember the difference between the two, but this explanation doesn't neatly cover all uses.
Personal and factual statements
A slightly less intuitive but more encompassing explanation is that 不 is used to negate statements which are based on personal circumstances, whereas 没 is used to negate statements of fact.
For example:
- 我 没 去 (I didn't go – an objective statement not reliant on personal views, it is a historical fact)
- 我 不 去 (I won't go – a decision based on personal circumstances. I won’t go because I don't have the money, because I'm busy or so on. This decision is up to me)
- 我 今天没 吃饭 (I haven't eaten today. This is a factual statement)
- 我 今天不 吃饭 (I'm not eating today. This is an opinion based on personal reasons; perhaps the speaker is feeling ill, is too busy or is too lazy. Again, this is my decision)
With this clear, it is now easier to explain another two uses of 不 and 没有
First, 没 is always used to negate 有. 有 means to be exist; when we state that things exist in everyday life, it is a factual statement (for example, "饼干没有了" "there are no biscuits" is a fact . For this reason we cannot negate 有 with 不.
Second, statements with negating adjectives are generally personal statementst, therefore we do not use 没 to negate adjectives. We can only say "不好" or "不漂亮", not "没好" or "没漂亮".
The examples below are a little more difficult, as these statement are very similar. However the same rule applies:
- 水 没 凉 (the water hasn't cooled down yet – this emphasizes that the objective process of water cooling hasn't finished yet, as it hasn't yet reached room temperature)
- 水不 凉 (the water isn't cool – here the speaker is making statement based on their belief of what cool is. I might think this water is cool, but you may not, the standard we use is subjective)
- 我 不 是 这个 意思 (No, you've got me wrong, you've judged the situation wrongly - a personal judgement on my behalf that the listener has misjudged me)
- 我 没有 这个 意思 (I never implied that - an objective fact)
