The negation 不 bù changes to bú (second tone) when followed by a fourth-tone syllable. Examples: 不去 búqù (not go), 不好 búhǎo (not good).
Tips & Tricks
Tips for learning Chinese: tones, phonetics, writing, and more.
不 bù before fourth tone becomes second tone (bú)
Two third tones in a row: the first becomes second tone
When two consecutive syllables both have third tone, the first changes to second tone. Examples: 你好 nǐhǎo → níhǎo (hello), 老鼠 lǎoshǔ → láoshǔ (mouse).