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.
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 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)
- 我今天没去吃饭 (I haven't eaten today. This is a simple objective statement)
- 我今天不去吃饭 (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)
From this point of view, it is now easier to explain another two uses of 不 and 没有
First, 没 is always used to negate 有. When we state that things exist in everyday life, it is a fact, as the glass on the table is either there or not. We cannot negate 有 with 不.
没有 is often used to state the loss or absence of something:
- 工作没了，钱没了！
- 饼干在哪儿？ 没有了，都吃完了！
Again, the fact that the biscuits have been eaten is a statement of fact, so we use 没（有）
Second, when we make negative judgments about things, we use 不, as this is a statement based on personal circumstances.
- 我觉得白酒不好喝
The examples below are a little more difficult, as these statement are very similar. However the same rule applies:
Some further examples:
- 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)
