Tone change rule

The stubborn 不

不 (bù) flips to bú before another falling tone.

Written

Spoken

bù + 4th tone is spoken as bú + 4th tone

不 is normally a falling tone. But two falls in a row sound like slamming two doors, so before another fourth tone, 不 bounces up instead. Before anything else it stays its stubborn falling self.

Some books print the change, some don't. We print bù everywhere and trust your ears to bounce it, exactly like native speakers learned to.

Words you already know, doing it

不是 bù shì bú shì is not

不要 bù yào bú yào don't

不对 bù duì bú duì not right

不去 bù qù bú qù not going

不忙 bù máng bù máng not busy (no change: máng is 2nd tone)