ChinesePathThe Yi and Bu Tone Change Rules (一 and 不)
Two Chinese words — and only these two — have their own personal tone change rules: 一 yī (one) and 不 bù (not). The rule is the same for both: before a fourth-tone syllable, they are pronounced with a second (rising) tone — 不去 is said bú qù, 一定 is said yí dìng. In every other position, 不 keeps its fourth tone, and 一 is pronounced with a fourth tone before first, second and third tones. As always in Mandarin, the pinyin spelling keeps the original tone: you write yī and bù, and only the pronunciation changes.
This guide gives you the complete rules, the reason behind them, and the special case where both words lose their tone entirely.
How does 不 (bù) change tone?
不 is a fourth-tone word (contour 62 — a sharp fall from the top). When the next syllable is also fourth tone, pronouncing two steep falls back to back is exhausting and sounds harsh — like an argument. Since 不 is one of the most frequent words in the language, speakers long ago smoothed it out: *before a fourth tone, 不 is pronounced bú (second tone)*. A rise followed by a fall is the most natural possible sequence.
| Written | Pronounced | Word |
|---|---|---|
| bù qù | bú qù | 不去 (not go) |
| bùshì | búshì | 不是 (is not) |
| bùcuò | búcuò | 不错 (not bad) |
| bùbì | búbì | 不必 (no need) |
| bùkàn | búkàn | 不看 (not look) |
| bùbiàn | búbiàn | 不变 (unchanging) |
Before first, second or third tones — and at the end of a sentence, or standing alone — 不 stays fourth tone: 不吃 bù chī, 不来 bù lái, 不好 bù hǎo, 不管 bù guǎn, 不！决不！bù! jué bù!
One more detail: this courtesy is exclusive to 不. No ordinary fourth-tone word changes before another fourth tone — 电脑 diànnǎo, 注意 zhùyì and 变化 biànhuà all keep their double falls. 不 earned its private rule purely through frequency.
How does 一 (yī) change tone?
一 is a first-tone word (contour 55 — high and flat), and its rules have one more branch than 不's:
| Position | Pronounced as | Examples (written → pronounced) |
|---|---|---|
| Before a 4th tone | 2nd tone (yí) | 一定 yīdìng → yídìng; 一次 yīcì → yícì; 一件 yījiàn → yíjiàn; 一律 yīlǜ → yílǜ |
| Before a 1st, 2nd or 3rd tone | 4th tone (yì) | 一天 yītiān → yìtiān; 一年 yīnián → yìnián; 一起 yīqǐ → yìqǐ; 一生 yīshēng → yìshēng |
| Counting, ordinal numbers, or final position | original 1st tone (yī) | 一 二 三 yī èr sān; 住在一楼 zhù zài yī lóu (live on the first floor); 长短不一 chángduǎn bù yī |
Four-character phrases follow the same logic, applied twice: 一路顺风 yīlùshùnfēng is pronounced yílùshùnfēng, and 一唱一和 yīchàngyīhè comes out as yíchàngyíhè.
Notice the elegant symmetry: although 一 and 不 start from different tones, their behavior converges — both become second tone before a fourth tone, and both are (or become) fourth tone before everything else.
When do 一 and 不 lose their tone completely?
Inside two common sandwich structures, both words shrink to a neutral tone — short, weak, unstressed, written without any tone mark:
- Reduplicated verbs with 一: 看一看 kàn yi kàn (take a look), 听一听 tīng yi tīng (have a listen), 摸一摸 mō yi mō (feel it)
- A-not-A questions and verb-complement pairs with 不: 吃不吃 chī bu chī (eat or not?), 好不好 hǎo bu hǎo (OK?), 推不动 tuī bu dòng (can't push it open), 起不来 qǐ bu lái (can't get up)
Does the pinyin spelling change?
No. Dictionaries and textbooks always write the original tones — yī and bù — regardless of pronunciation. When course materials (including the tables above) print yí, yì or bú, that is a pronunciation aid, not standard spelling. This is the same convention as third tone sandhi: tone changes live in speech, never in writing.
Frequently Asked Questions
Why do 一 and 不 change tone but other words don't?
Frequency. Both words appear constantly in speech, so the awkward tone sequences they create (especially two fourth tones in a row) got smoothed out by generations of speakers. Ordinary words are not frequent enough to have earned this treatment — 电脑 (diànnǎo) keeps both fourth tones.
Is the change to second tone before a fourth tone optional?
No. Saying 不是 with two full fourth tones sounds unnatural and harsh to native ears. The change is a fixed rule of standard Mandarin, not a stylistic choice.
How should I write 一 and 不 in pinyin?
Always with their original tones: yī and bù. Standard spelling never records the sandhi — yí ge and bú shì are pronunciation transcriptions, not correct pinyin spellings.
When does 一 keep its first tone?
When counting (yī, èr, sān), in ordinal uses (一楼 yī lóu, the first floor), and at the end of a word or phrase (长短不一 chángduǎn bù yī). In these positions there is no following syllable driving a change.
Do 一 and 不 follow the same rule?
Essentially yes: before a fourth tone both are pronounced second tone; before other tones both are pronounced fourth tone (不 already is one; 一 changes to it). Remembering them as one shared pattern halves the memory load.
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