1. 不 is a 4th tone syllable by itself.
2. The tone doesn’t change when 不 is followed by a 1st tone, 2nd tone or 3rd tone syllable.
|不 + 1st Tone||不吃 （bù chī）|
|不 + 2nd Tone||不行 （bù xíng）|
|不 + 3rd Tone||不想 （bù xiǎng）|
3. 不 becomes the 2nd tone when it’s followed by a 4th tone syllable.
|不 + 4th Tone||不要 （bú yào）|
4. When 不 is in the between of “A 不 A” pattern phrase，不 will be the neutral tone.
|A 不 A||好不好 （hǎo bu hǎo）|