Tone Change

不 (bu), meaning "no" or "not" is normally pronounced in the fourth tone.

However, it is pronounced in the second tone when it is used before a character using the fourth tone as well.

Examples:

The character following 不 doesn't use the fourth tone, therefore 不 keeps the fourth tone.

不能 (bù néng) = cannot
不可以 (bù kěyǐ) = not allowed
不行 (bù xíng) = be unable to
不喜欢 (bù xǐhuān) = don't like

The character following 不 uses the fourth tone, therefore 不 uses the second tone.

不对 (bú duì) = not correct
不去 (bú qù) = don't go
不是 (bú shì) = is not
不要 (bú yaò) = don't want to

The fourth tone is indicated by the number 4 (bu4) or a dropping line (bù).
The second tone is indicated by the number 2 (bu2) or a rising line (bú).