不 (bù) is the negator you'll say hundreds of times a day, and it carries one small tone habit that instantly marks fluent speech: it changes tone depending on what follows. On its own and before most syllables it is a falling 4th tone, bù — but before another 4th tone it flips up to a rising 2nd tone, bú. This page is the negation-relevant summary: which words you'll actually be negating trigger the shift, and how to drill it. For the full phonology — why the change happens, the parallel with 一, and the writing conventions — see the dedicated 不 sandhi page.
The rule in one line
不 → bú before a 4th tone; bù everywhere else. The character 不 never changes — only its pronunciation does, automatically, every time. Many of the most common negations are 不 + a 4th-tone syllable, so you'll meet the rising bú constantly:
我不是老师。
wǒ bú shì lǎoshī
I'm not a teacher. — 是 (shì) is 4th tone, so 不 → bú. (neutral)
我不要，谢谢。
wǒ bú yào, xièxie
I don't want it, thanks. — 要 (yào) is 4th tone, so 不 → bú. (informal)
这个不对。
zhège bú duì
This isn't right. — 对 (duì) is 4th tone, so 不 → bú. (informal)
The 4th-tone verbs and adjectives you'll negate most
Because so many everyday verbs and adjectives end up under negation, it helps to notice which ones are 4th-tone (they take bú) and which aren't (they keep bù):
| Negation | Following tone | 不 is pronounced |
|---|---|---|
| 不是 (is not) | shì — 4th | bú shì |
| 不去 (not go) | qù — 4th | bú qù |
| 不要 (don't want) | yào — 4th | bú yào |
| 不错 (not bad / pretty good) | cuò — 4th | bú cuò |
| 不累 (not tired) | lèi — 4th | bú lèi |
| 不喝 (not drink) | hē — 1st | bù hē |
| 不来 (not come) | lái — 2nd | bù lái |
| 不好 (not good) | hǎo — 3rd | bù hǎo |
我今天不做饭。
wǒ jīntiān bú zuò fàn
I'm not cooking today. — 做 (zuò) is 4th tone, so 不 → bú. (informal)
这家店不错，下次再来。
zhè jiā diàn bú cuò, xià cì zài lái
This shop's pretty good — let's come again. — 不错 is bú cuò. (informal)
我一点都不累，接着走吧。
wǒ yìdiǎn dōu bú lèi, jiēzhe zǒu ba
I'm not tired at all — let's keep walking. — 累 (lèi) is 4th tone, so bú lèi. (informal)
Contrast the ones that stay bù, where the verb happens to start on a non-4th tone:
我不喝咖啡，晚上睡不着。
wǒ bù hē kāfēi, wǎnshang shuì bu zháo
I don't drink coffee — I can't sleep at night. — 喝 (hē) is 1st tone, so 不 stays bù. (informal)
When 不 goes neutral: A-不-A and potential forms
There is one place 不 does something else in a negation-shaped structure: when it's squeezed between two syllables, it drops to a light neutral tone (written bu, no mark). This happens in A-不-A questions and in potential complements:
你要不要？
nǐ yào bu yào?
Do you want it or not? — the middle 不 is light and neutral. (informal)
这个字我看不懂。
zhège zì wǒ kàn bu dǒng
I can't read this character. — potential 看不懂, 不 goes neutral. (informal)
Here the 不 isn't fully bù or bú — it's an unstressed bu. For how the A-不-A question is built, see the A-not-A page.
A note on writing and on 一
Two things worth knowing. First, unlike third-tone sandhi, the 不 change is usually written into the pinyin — textbooks print bú shì, not bù shì — so a bú on the page is showing you the real, changed sound. Second, don't confuse 不 with the number 一 (yī), which sandhis on a busier schedule (yì before 1st/2nd/3rd, yí before a 4th tone). They rhyme but aren't the same rule — see 一 sandhi.
Common Mistakes
1. Saying a flat bù before a 4th-tone verb. The signature beginner tell: bù qù, bù shì, bù yào with two falling tones. Understood, but distinctly non-native.
❌ 不去 pronounced bù qù
Incorrect — 去 is 4th tone, so 不 must rise: bú qù.
✅ 不去 pronounced bú qù
bú qù
not going — 不 rises to bú before the 4th-tone 去.
2. Over-applying the rise. Only a following 4th tone triggers bú. Before 1st/2nd/3rd, 不 stays bù.
❌ 不喝 pronounced bú hē
Incorrect — 喝 is 1st tone, not 4th, so 不 stays bù.
✅ 不喝 pronounced bù hē
bù hē
not drinking — 不 keeps its falling tone before the 1st-tone 喝.
3. Thumping the neutral 不 in A-不-A. In 要不要, 去不去, the middle 不 is light — don't stress it as bù or bú.
❌ 去不去 pronounced qù bú qù
Marginal — the middle 不 should be a light neutral bu, not a stressed bú.
✅ 去不去 pronounced qù bu qù
qù bu qù
going or not? — the middle 不 is neutral.
4. Thinking the tone change alters the meaning. bù and bú are the same word 不 ("not") — only the tone shifts for euphony. Don't hunt for a difference in sense.
Key Takeaways
- 不 → bú (2nd tone) before a 4th tone (不是 bú shì, 不去 bú qù, 不要 bú yào, 不对 bú duì); bù everywhere else (不喝 bù hē, 不来 bù lái, 不好 bù hǎo).
- The character never changes — it's a sound habit, applied automatically every time.
- Many of the commonest negations are 不 + a 4th-tone verb, so the bú form is everywhere.
- In A-不-A questions and potential forms (要不要, 看不懂), the sandwiched 不 goes neutral (bu).
- For the full phonology and the 一 parallel, see the pronunciation page.
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- 不 Sandhi (bù → bú)HSK 1 — The negator 不 is normally a 4th tone (bù), but it flips to a rising 2nd tone (bú) whenever the next syllable is also 4th tone — 不是 bú shì, 不要 bú yào — while staying bù before 1st, 2nd, and 3rd tones; it's a fixed euphonic rule, not a choice, and unlike third-tone sandhi it's usually written into the pinyin.
- 没(有) (méi yǒu): Negating 有 and Completed ActionHSK 1 — 没(有) does two jobs: it is the only way to negate 有 (有 is never 不有), and it negates completed or experienced actions — 我没吃 'I didn't eat', dropping the 了. Think of 没 as the completion/existence/change negator.
- A-not-A Questions (是不是, 去不去)HSK 1 — A second neutral way to ask yes/no: juxtapose the positive and negative of the verb or adjective — 去不去, 好不好, 是不是. The middle 不 is toneless, 有 uses 没 (有没有), and you never add 吗.
- 一 Sandhi (yī → yí / yì)HSK 2 — 一 'one' is 1st tone in isolation, but in running speech it flips its tone to avoid clashing with the syllable that follows — 2nd tone (yí) before a 4th tone, 4th tone (yì) before a 1st, 2nd, or 3rd tone — while staying plain yī in ordinals and number strings.