The Yi and Bu Tone Change Rules, Explained Once and for All

If you searched for the yi and bu tone change rules, here is the complete answer up front. 一 (yī, "one") is first tone in isolation, but it becomes yí (second tone) before a fourth-tone syllable — 一个 (yí ge) — and yì (fourth tone) before first, second, or third tones — 一天 (yìtiān), 一年 (yìnián), 一起 (yìqǐ). It stays yī when counting and in ordinals like 第一 (dì-yī). 不 (bù, "not") is simpler: it becomes bú (second tone) before a fourth tone — 不是 (bú shì) — and stays bù everywhere else.

That is the whole system. The rest of this guide makes it stick: why the changes happen, the exceptions examiners love, worked examples with hanzi and pinyin, and a practice routine that moves the rules from your notes into your mouth.

Hear it, don't just read it: record yourself saying 不是 (bú shì) and 一个 (yí ge) with the free AI pronunciation checker — it scores every syllable's tone, so you'll know in 30 seconds whether your sandhi is real or theoretical.

What "tone change" (tone sandhi) actually means

Mandarin tones are not always pronounced the way the dictionary lists them. When certain tones bump into each other in connected speech, one of them changes. Linguists call this tone sandhi, and Mandarin has three rules that matter for learners:

1. Third tone sandhi — a third tone before another third tone becomes second tone: nǐ hǎo 你好 is actually pronounced ní hǎo. This is the big one, and we cover it in depth in our guide to third tone sandhi rules.
2. The 一 (yī) rule — the word for "one" changes tone depending on what follows it.
3. The 不 (bù) rule — the negation word changes tone before fourth tones.

Third tone sandhi applies to every third-tone syllable in the language. The 一 and 不 rules are different: they apply to exactly two words. That sounds easier — and it is — but because 一 and 不 are among the most frequent words in Mandarin, you will use these rules in almost every sentence you ever say. Get them wrong and you sound persistently, noticeably off; get them right and your Mandarin suddenly flows.

The base tones: yī and bù

Before the changes, know the defaults:

• 一 (yī) — first tone, high and flat. This is its citation form: when you count 一、二、三 (yī, èr, sān), read a phone number, or say it at the end of a phrase, it is yī.
• 不 (bù) — fourth tone, sharp and falling. Alone, or before most syllables, it is bù.

Dictionaries always list these base forms. The sandhi forms below are what happens in real speech.

Rule 1: 一 becomes yí before a fourth tone

When the syllable after 一 carries a fourth tone, 一 flips to second tone (yí). Two falling tones in a row are awkward to produce, so the language dissimilates the first one into a rise.

Note 一个 (yí ge): the measure word 个 (gè) is usually pronounced with a neutral tone, but its underlying tone is fourth — and sandhi reacts to the underlying tone. That is why "one of something," probably the most common 一 phrase in the language, is yí ge.

Rule 2: 一 becomes yì before first, second, and third tones

When the next syllable is first, second, or third tone, 一 becomes fourth tone (yì):

A compact way to memorize both rules: 一 always moves away from the tone that follows it. Before a falling tone it rises (yí); before anything else it falls (yì). If you remember that single sentence, you can reconstruct every example above.

The exceptions: when 一 stays yī

一 keeps its original first tone whenever it functions as a plain number rather than a modifier of what follows:

• Counting and digits: 一、二、三 (yī, èr, sān); phone numbers; room 301 read as sān líng yāo/yī.
• Ordinals: 第一 (dì-yī, "first"), 第一名 (dì-yī míng, "first place"), 一月 (yīyuè, "January"), 一号 (yī hào, "the 1st" of the month).
• Ends of phrases: 十一 (shíyī, "eleven"), 唯一 (wéiyī, "the only"), or 一 read out alone.
• Years read digit by digit: 二〇二一年 (èr líng èr yī nián).

The logic: sandhi is triggered by the relationship between 一 and the word it quantifies. In 一个 it is "one of something," so the rule fires. In 第一 or 十一, 一 is just naming a position or a digit — nothing follows for it to lean on, so it stays yī. (Bonus: in phone numbers and room numbers, native speakers often replace yī with yāo entirely, to avoid mishearing.)

Rule 3: 不 becomes bú before a fourth tone

不 has exactly one change. When the next syllable is fourth tone, bù dissimilates into bú (second tone) — same logic as 一: the language avoids two hard falls in a row.

Before first, second, and third tones, 不 keeps its fourth tone:

• 不高 (bù gāo) — not tall (1st tone follows)
• 不忙 (bù máng) — not busy (2nd tone follows)
• 不好 (bù hǎo) — not good (3rd tone follows)

Because 不是 (bú shì) is one of the most frequent two-syllable chunks in Mandarin, this single rule fixes a mistake you would otherwise make dozens of times per conversation. Drill the word on its own pronunciation page for 不 (bù), then in phrases.

Bonus rule: 不 goes neutral in the middle

In two very common patterns, 不 weakens all the way to a neutral tone (written bu, light and quick):

1. A-not-A questions: 去不去？(qù bu qù? — "going or not?"), 是不是 (shì bu shì), 好不好 (hǎo bu hǎo).
2. Verb + 不 + complement: 看不懂 (kàn bu dǒng — "read but not understand"), 买不起 (mǎi bu qǐ — "can't afford"), and the fossilized 对不起 (duìbuqǐ — "sorry").

This is a politeness of rhythm rather than a hard rule — a full bù is understandable — but the neutral version is what you will hear from every native speaker.

Worked examples: reading real sentences

Apply everything to full sentences. Base tones on the left, what you actually say on the right:

1. 我买了一件一样的衣服。 → Wǒ mǎi le yí jiàn yíyàng de yīfu. (Both 一 precede 4th tones → yí.)
2. 他一天学一百个词，不累吗？ → Tā yìtiān xué yìbǎi ge cí, bú lèi ma? (一 before 1st and 3rd → yì; 不 before 4th → bú.)
3. 这是我第一次来，一切都不一样。 → Zhè shì wǒ dì-yī cì lái, yíqiè dōu bù yíyàng. (Ordinal 第一 stays yī; 一切 has 4th tone qiè → yí; 不 before yí — now a second tone — stays bù; 一样 → yíyàng.)
4. 你去不去？我不去。 → Nǐ qù bu qù? Wǒ bú qù. (Neutral in the A-not-A, bú before the 4th-tone verb.)

Example 3 shows the subtlety worth internalizing: sandhi chains read left to right off the surface tone. In 不一样, 一 changes first (yíyàng), and 不 then sees a second tone — so it stays bù: bù yíyàng.

How to actually learn this (not just know it)

Knowing the rules and producing them at speed are different skills. Rules live in your slow, deliberate memory; speech happens too fast to consult it. The fix is to train chunks, not rules:

1. Memorize the six anchor phrases — 一个 (yí ge), 一样 (yíyàng), 一起 (yìqǐ), 一天 (yìtiān), 不是 (bú shì), 不对 (bú duì). These cover the whole rule set. When in doubt mid-sentence, analogize to the anchor.
2. Drill the isolated words first. Get a clean yī on the 一 (yī) pronunciation page and a sharp bù on the 不 (bù) page, so the sandhi versions have a stable base to deviate from.
3. Record and verify. Say the anchor phrases into the free tone checker. The AI grades each syllable's tone contour, so "I think that was yí" becomes a score. Most learners discover their "yí" is actually a flat yī within the first two recordings.
4. Read pinyin out loud with base tones printed. Since most texts print yī and bù unchanged, practicing the conversion while reading is exactly the skill you need in the wild.
5. Slot it into a bigger tone routine. Sandhi is one layer of the tone system; pairs, contours, and listening come first. Our complete guide to learning Chinese tones shows where these rules fit in the overall progression.

Common mistakes (and how to catch them)

Four errors account for almost every 一/不 problem we hear in learner recordings:

1. Dictionary-mode reading. You practice from pinyin that prints yī and bù, so your mouth memorizes the base tones and repeats them everywhere — 不是 comes out as bù shì every single time. Fix: whenever you meet 一 or 不 in a text, scan the next syllable's tone before you say the word. Slow at first, automatic within days.
2. Overcorrection. After learning the rule, many learners start saying yí everywhere — including 一起 (yìqǐ) and 一天 (yìtiān), where 一 must fall, not rise. If every 一 in your speech became yí, you learned half the rule.
3. Hypercorrected ordinals. 第一 (dì-yī) said as dì-yí because "一 changes before other syllables, right?" Ordinals and plain numbers are the exception — keep them first tone.
4. Missing the underlying tone. In 一个, the 个 sounds neutral, so learners reason "no fourth tone follows, 一 stays yī." But sandhi reads the underlying fourth tone of 个 (gè) — the correct form is yí ge.

Notice the pattern: in all four cases your knowledge is fine and your reflexes lag. That is a feedback problem, not a study problem — which is why recording yourself and checking the scores beats re-reading the rules for the fifth time.

The one-paragraph summary

一 is yī by default and in numbers/ordinals (第一 dì-yī, 十一 shíyī, 一月 yīyuè). Before a fourth tone it becomes yí (一个 yí ge, 一样 yíyàng); before first, second, or third tones it becomes yì (一天 yìtiān, 一年 yìnián, 一起 yìqǐ). 不 is bù by default and becomes bú only before fourth tones (不是 bú shì, 不要 bú yào); in A-not-A and verb-complement patterns it reduces to neutral bu (去不去 qù bu qù, 对不起 duìbuqǐ). Both changes exist to avoid back-to-back falling tones — and both are judged by the surface tone of the next syllable.

Reading the rules took you five minutes. Making them automatic takes about a week of two-minute drills — if you get feedback. Open the free AI pronunciation test, say 一个 (yí ge), 一起 (yìqǐ), and 不是 (bú shì), and see whether your tones actually change. That closed loop — say it, score it, fix it — is how 20,000+ learners are training tones with TonePerfect.