4. Tone Combination (2)

Tone Change rules related to characters

一 (yī, one)	一定 (yí dìng, certainly) 一天 (yì tiān, one day)
不 (bù, no)	不是 (bú shì, not) 不要 (bú yào, don't)

When followed by a 4th tone, 不 (bù) changes to 2nd tone (bú).