L1p37

Rules of the Pinyin (phonetics)<5>

Another often used word “不”(meaning “No” or “Not”)has two tones:

A. When it is followed by a word with the first,second or third tone, it pronounces as “bù” (the fourth tone).

For example:

不吃 -- bù chī (meaning “not eat”)     

不能 –- bù néng (meaning “not able”)     

不想 –- bù xiǎng (meaning “not think”)     

B. When it is followed by a word with the fourth tone, it pronounces as “bú” (the second tone).

For example:

不客气 –- bú kè-qi (meaning “you are welcome”)     

不怕 –- bú pà (meaning “not afraid”)