The Tone Changes of bù (不)

After 一, the second word that bends the tone rules is 不 (bù) — “not”, the all-purpose negator you’ll use in nearly every sentence. The bù tone change is even simpler than the rule for 一: 不 has exactly one exception. Once you have it, negation stops tripping up your tones.

The default: bù (falling)

In isolation and in front of most syllables, 不 (bù) is a plain fourth tone — a sharp fall from high to low. This is its dictionary tone, and it stays this way before a first, second, or third tone:

Phrase	Following tone	Spoken	Meaning
不高 (bù gāo)	1st	bù gāo	”not tall”
不来 (bù lái)	2nd	bù lái	”not coming”
不好 (bù hǎo)	3rd	bù hǎo	”not good”
不能 (bù néng)	2nd	bù néng	”cannot”

So most of the time, 不 is just bù. You only have to remember the single situation where it changes.

The one exception: before a fourth tone → bú (rising)

When 不 is immediately followed by another fourth tone, it rises to a second tone, bú. Two sharp falls in a row are awkward, so the first one lifts up instead:

Phrase	Following tone	Spoken	Meaning
不是 (bù shì)	4th	bú shì	”is not”
不对 (bù duì)	4th	bú duì	”not right; wrong”
不去 (bù qù)	4th	bú qù	”not going”
不要 (bù yào)	4th	bú yào	”don’t want; don’t”

These are extremely common phrases, so the rising bú ends up being one of the sounds you produce most often. Say 不是 (bú shì) and 不对 (bú duì) until the upward lift feels automatic.

A neat way to hear it: 不 dissolves a clash of two falling tones, the same instinct behind third-tone sandhi dissolving two dipping tones. Mandarin keeps nudging awkward back-to-back tones apart.

The A-不-A question pattern

There’s one more place 不 shows up constantly: the “verb-不-verb” question, a core beginner structure for asking yes/no questions. The 不 sits between two copies of a word, and its tone follows the same rule — it depends on the tone of whatever comes right after it:

• 是不是 (shì bú shì) — “is it or not?” — 不 is before a fourth tone (是), so it rises to bú
• 对不对 (duì bú duì) — “right or not?” — again before a fourth tone, so bú
• 好不好 (hǎo bù hǎo) — “okay?” — 不 is before a third tone (好), so it stays bù
• 来不来 (lái bù lái) — “coming or not?” — before a second tone, so bù

In fast, casual speech the middle 不 is often shortened to a light neutral tone, but the underlying rule still tells you which way it leans.

How 不 differs from 一

不 and 一 are the two famous tone-changing function words, and it’s worth seeing them side by side so you don’t mix up their rules:

	Default tone	Changes when followed by	Changes to
一 (yī)	1st (yī)	1st/2nd/3rd → yì · 4th → yí	both directions
不 (bù)	4th (bù)	4th only → bú	one direction

The key contrast: 一 changes in two directions, depending on what follows, while 不 changes in only one — it rises to bú solely before a fourth tone and is otherwise left alone. That makes 不 the easier of the pair. Pinyin usually writes the changed tone for both (bú shì, yí gè), so the spelling tends to match the sound — unlike the hidden change in third-tone sandhi.

A quick contrast drill

Pairing a “changing” phrase with a “stable” one trains the difference fast:

• 不是 (bú shì) “is not” vs. 不好 (bù hǎo) “not good”
• 不要 (bú yào) “don’t” vs. 不能 (bù néng) “cannot”

Once 一 and 不 feel automatic, the remaining challenge is stringing several third tones together, which we cover in multiple third tones and chunking.