--description--

Tone Sandhi of 不 (bù)

不 (bù) means "no" or "not". When it is followed by a syllable which is in the first, second or the third tone, it remains in the fourth tone.

However, when 不 (bù) is followed by a syllable which is in the fourth tone, it changes to the second tone.

This tone change is a natural adjustment in speech to make pronunciation smoother.

Examples

Written	Pronounced as
bù néng	bù néng
bù xiǎng	bù xiǎng
bù shì	bú shì
bù huì	bú huì

--assignment--

I've read this Pinyin rule.