不 Sandhi (bù → bú)

The word 不 (bù) — "not," Mandarin's main negator — is one of the most frequent words in the language, so its one small tone change is worth getting right early. On its own and before most syllables, 不 is a falling 4th tone: bù. But whenever the very next syllable is also a 4th tone, 不 dodges the clash and shifts up to a rising 2nd tone: bú. So "not" changes shape depending on the tone of whatever it is negating. This is a fixed euphonic rule — not a stylistic option, not something only fast speakers do — and native speakers apply it every single time. For what 不 actually means and how it differs from 没, see 不 vs 没; this page is purely about its tone.

The rule: bù → bú before a 4th tone

Two falling tones in a row are hard on the voice — you would have to drop from high to low, then leap back up to high only to fall again. Mandarin smooths this by turning the first faller into a riser. So bù + [4th tone] → bú + [4th tone]:

In each case the following syllable is a full 4th tone (shì, yào, duì), so 不 climbs into a rising bú. Say it and feel the logic: your voice glides up into bú, then falls sharply on the next word — a smooth up-then-down instead of an awkward down-then-down.

Before every other tone, 不 stays bù

The change only happens before a 4th tone. Before a 1st, 2nd, or 3rd tone, 不 keeps its normal falling bù. There is no half-measure and no rise:

So the full picture is neat: 不 is bú before a 4th tone and bù everywhere else. You can check any word by asking one question — "what tone comes next?" — and if the answer is "4th," flip it to bú.

In real sentences

The rule fires constantly in ordinary speech, since 不 negates verbs and adjectives all day long:

Note the last example also shows a second thing: inside 对不起 (duìbuqǐ, "sorry"), the 不 sits between two syllables and is squeezed down to a neutral tone — light and toneless — rather than either bù or bú. That reduction happens in a handful of set expressions and in A-不-A questions (below); elsewhere the 4th-tone/2nd-tone rule above is what you need.

The A-不-A question: 不 goes neutral

Mandarin forms a common yes-no question by sandwiching 不 between a repeated verb or adjective: 是不是 (shì bu shì, "is it or isn't it?"), 对不对 (duì bu duì, "right?"), 去不去 (qù bu qù, "are you going?"). In this pattern the 不 is unstressed and typically neutral (toneless), written bu:

The contrast with third-tone sandhi

Two sandhi rules, one important difference in how they're written. Third-tone sandhi (你好 → ní hǎo) is never written — pinyin keeps the underlying nǐ hǎo and trusts you to change it. The 不 rule, by contrast, is usually written into the pinyin: textbooks and dictionaries print bú shì, not bù shì. The same is true of the number 一 (一 sandhi), whose changes are also written out. So when you read pinyin: a bú on the page is showing you the real, changed pronunciation, while a 3rd-tone mark on the page is showing you the underlying tone that you must change yourself. Two conventions, and knowing which is which saves a lot of confusion.

Common Mistakes

1. Keeping bù (4th tone) before another 4th tone. The core error: saying bù shì, bù yào, bù duì with two falling tones. It is grammatically understood but sounds distinctly foreign, because native speech never leaves that clash unresolved.

2. Over-applying the change. The opposite mistake: raising 不 to bú before a 1st, 2nd, or 3rd tone. The rule fires only before a 4th tone; everywhere else 不 stays a falling bù.

3. Forgetting the neutral 不 in A-不-A questions. In 去不去, 对不对, 是不是, the middle 不 is light and toneless — not a stressed bù or bú. Thumping it there sounds unnatural.

4. Assuming the change means the meaning changed. bù and bú are the same word 不 with the same meaning ("not"); only the tone shifts for euphony. Don't hunt for a difference in sense — there isn't one.

Key Takeaways

• 不 is normally a falling bù, but becomes a rising bú before another 4th tone (不是 bú shì, 不要 bú yào, 不对 bú duì).
• Before 1st, 2nd, and 3rd tones it stays bù (不多 bù duō, 不难 bù nán, 不好 bù hǎo).
• It's a fixed euphonic rule, applied every time — not optional.
• In A-不-A questions (去不去, 对不对), the middle 不 goes neutral (toneless).
• Unlike third-tone sandhi, the 不 change is usually written in pinyin — a bú on the page shows the real sound.