The rule of tone sandhi for NO-不( bù )
(1) 不 the original pronounce is the fourth tone in Chinese. Thus, there have three situations have to pronounce as a fourth tone.
(i) 不( bù ) -Single Chinese character
(ii) 從不(cóng bù )/絕不(jué bù) –Suffix
(iii) 不該(bù gāi )/不來(bù lái)/不管(bù guǎn) – If the 不 is in front of the first tone, second tone and third tone of Chinese characters.
(2) If 不 is in front of the fourth tone of Chinese character, then不have to pronounce as a second tone.
不要(bú yào)/不用(bú yòng)/不對(bú duì)
(3) If 不 in the middle of overlap verbs or adjective, then不have to pronounce as a natural tone.
要不要(yào bu yào)/可愛不可愛(kě ài bu kě ài)