不 is 4th tone and is used to negate all verbs apart from one. Examples: 不行！This is not OK 这个不好 - This is not good 我不想去 - I don't want to go However, when 不 appears before a 4th tone word, it changes to 2nd tone. Examples: 他不是老师 - He is not a teacher 我不去 - I'm not going 他不在 - He's not here 不要！- Don't (do that)! 这样是不对的 - Doing it this way is wrong Phrases like 不对 and 不是 are very common. I've found that the best thing to do is to memorise the tones of these phrases, and not so much the individual characters. Obviously you want to know the tones of the characters, but when you speak you often don't have time to think of the spot - which is why learning the tones in phrases such as these is useful. Now, there is one exception I know to this. When you say "为什么不？", meaning "why not", 不 here is 1st tone. When you 呢 to the end - 为什么不呢？, it changes back to 4th tone. To negate 有, you use 没. 有 means "to have" or "there is/are" 我有笔 - I have a pen 她有很多朋友 - She has lots of friends 家里有人 - There are people at home 我没有笔 - I don't have a pen 她没有朋友 - She doesn't have any friends 家里没有人 - There isn't anybody at home I hope this is helpful. Laurence

“不”actually has only one original tone (bù)according to dictionary，like 不行、不好 【注意】 由于语音的原则，在四声字的前面，就读二声；如，不bú去qù 在一二三声字的前面，就读四声。如，不屈 不明 不好；后面为四声，就读二声。

It's Bù (4th)

Thank you, mavis

actually, this question related to Chinese's grammar. it's called Pronunciation changes (or successive phonetic change ),which means the pronunciation of some characters would be changed if you put them before another characters, just like "不"and"一".in Chinese we called it 语音流变，I thought Laurence have given us so many good examples, but I didn't agree with him too much on that "when you say "为什么不?"meaning why not, here 不is 1st tone "normally, we also use 4th in this sentence instead of 1st ,to express the mood of rhetorical question. and when we want to express"为什么不这样呢?-why do we. ..we often use 1st "that's it, and Laurence really did a good job.