Tone Changes for 不 (bù)
How the negation word changes its tone
Category: Tones & Pronunciation
The Rule
不 (bù) is normally 4th tone, but changes to 2nd tone (bú) when followed by another 4th tone syllable.
Why This Matters
This is a phonetic convenience rule — two falling tones in a row are hard to pronounce smoothly. English doesn't have this kind of automatic sound change, so you need to consciously learn it. The good news: it's a simple, consistent rule with no exceptions.
Examples
• 不是 (bú shì) — "is not" [不 changes to 2nd tone before 4th tone 是] • 不对 (bú duì) — "incorrect" [不 changes to 2nd tone before 4th tone 对] • 不好 (bù hǎo) — "not good" [不 stays 4th tone because 好 is 3rd tone] • 不能 (bù néng) — "cannot" [不 stays 4th tone because 能 is 2nd tone]
Common Mistakes
❌ Always saying bù regardless of what follows ✅ Say bú before 4th tone words, bù before others → 不要 (bú yào, don't) and 不去 (bú qù, not going) both need the 2nd tone change because 要 and 去 are 4th tone. ❌ Changing 不 to 2nd tone before all tones ✅ Only change before 4th tone → 不吃 (bù chī, not eat) keeps 4th tone because 吃 is 1st tone. The rule ONLY applies before 4th tone.
Quick Tip
Quick test: is the next word 4th tone (falling)? If yes, 不 = bú. If no, 不 = bù.
Quick test: is the next word 4th tone (falling)? If yes, 不 = bú. If no, 不 = bù.
Examples
Common Mistakes
Incorrect: Always saying bù regardless of what follows → Correct: Say bú before 4th tone words, bù before others. 不要 (bú yào, don't) and 不去 (bú qù, not going) both need the 2nd tone change because 要 and 去 are 4th tone.
Incorrect: Changing 不 to 2nd tone before all tones → Correct: Only change before 4th tone. 不吃 (bù chī, not eat) keeps 4th tone because 吃 is 1st tone. The rule ONLY applies before 4th tone.
Quiz
How do you pronounce 不去 (not go)?