Tone sandhi explained: third tone, 不 and 一 made simple

The word sandhi comes from Sanskrit and means "joining". In Chinese, tone sandhi (变调 biàndiào) is when a tone changes depending on the tone that follows it.

It isn't a weird exception: it's like English connected speech, where "want to" becomes "wanna" so speech flows more smoothly. Chaining two full third tones (dip-rise, dip-rise) would be tiring, so the language simplified it.

There are three main rules to know, plus a fourth about the "half-third tone". They are automatic for native speakers: someone saying 你好 doesn't think "I'm changing the tone", they just do it.

How does pinyin show these changes? This causes a lot of confusion:

• For 不 and 一, textbooks and Hanlu usually write the tone actually spoken: 不是 bú shì, 一个 yí gè.

• For tone 3, the original tone is always written: 你好 is spelled nǐ hǎo but pronounced "ní hǎo". Applying the rule is up to you.

Dictionaries always give the base tone: 不 bù, 一 yī.

When two third-tone syllables follow each other, the first is pronounced as a second tone.

3 + 3 → 2 + 3

• 你好 nǐ hǎo → "ní hǎo"

• 很好 hěn hǎo → "hén hǎo"

• 可以 kěyǐ → "kéyǐ"

• 水果 shuǐguǒ → "shuíguǒ"

• 展览 zhǎnlǎn → "zhánlǎn"

This applies inside words and across words in the same rhythmic group: 我也 (wǒ yě) → "wó yě".

What about three or more third tones? It depends on how the sentence splits into meaning groups. In practice, the simplest rule: keep tone 3 on the last syllable of the group, and the earlier ones usually become tone 2.

• 我很好 wǒ hěn hǎo → "wó hén hǎo" (most common at normal speed)

• 展览馆 zhǎnlǎnguǎn (exhibition hall) → "zhán lán guǎn"

• 我想买 wǒ xiǎng mǎi → "wó xiáng mǎi"

Don't worry if you're unsure about grouping: at normal speed, both pronunciations are acceptable, and native speakers vary too.

Special case: when the second third tone becomes neutral. If the second syllable is a neutral tone that was originally a third tone, the rule is irregular: 姐姐 jiějie is said with a low tone 3 + high neutral, while 想想 xiǎngxiang is said "xiángxiang". Learn these words by ear.

不 (no, not) is tone 4: bù. But before another tone 4 syllable, it changes to tone 2: bú.

• 不是 → bú shì (is not)

• 不对 → bú duì (not right)

• 不要 → bú yào (don't want, don't)

• 不去 → bú qù (not going)

• 不客气 → bú kèqi (you're welcome)

Before tones 1, 2 or 3, 不 stays bù:

• 不喝 bù hē, 不来 bù lái, 不好 bù hǎo.

Why? Two falling tones in a row (fall, fall) sound jerky; tone 2 acts as a springboard.

不 in the neutral tone. In the middle of a verb-不-verb question or a resultative/potential complement, 不 is often neutral: 是不是 shì bu shì, 去不去 qù bu qù, 看不懂 kàn bu dǒng, 听不见 tīng bu jiàn.

Common mistake: ✗ "bù shì" with two heavy falling tones. ✓ "bú shì". It's one of the first errors a native speaker notices.

一 (one) is the most capricious character. Its base tone is tone 1: yī. But it changes with position:

a) yī (tone 1) when alone, at the end of a word, or in numbers/counting:

• 一 yī, 第一 dì-yī (first), 十一 shíyī (eleven), 一月 yīyuè (January), 一楼 yī lóu (1st floor), 一二三 yī èr sān.

b) yí (tone 2) before a tone 4, including before 个 gè, pronounced as tone 4 here:

• 一个 yí gè, 一样 yíyàng (the same), 一下 yíxià (a bit), 一定 yídìng (surely), 一共 yígòng (altogether).

c) yì (tone 4) before tones 1, 2 or 3:

• 一天 yì tiān (one day), 一年 yì nián (one year), 一起 yìqǐ (together), 一些 yìxiē (some), 一点儿 yìdiǎnr (a little).

d) yi (neutral) between reduplicated verbs:

• 看一看 kàn yi kàn (take a look), 想一想 xiǎng yi xiǎng (think it over).

Tip: 一 goes to tone 2 before tone 4, and to tone 4 before everything else. Remember the models 一个 yí gè and 一天 yì tiān: they cover 90% of cases.

If you listen carefully to native speakers, you'll notice that the full third tone (214, dip then rise) is rare. Here is the real rule:

• Tone 3 + tone 1, 2, 4 or neutral → half-third (21): go low and don't come back up. 老师 lǎoshī, 很忙 hěn máng, 可是 kěshì, 我们 wǒmen.

• Tone 3 + tone 3 → tone 2 (rule 1).

• Tone 3 at the end of a sentence or alone → full 214, sometimes even reduced in fast speech.

Practical consequence: in 90% of cases, your third tone should be low, deep, with no rise. That lowness is what sets it apart. Thinking "tone 3 = low" will make your pronunciation much more natural.

A good test: say 我是学生 (Wǒ shì xuésheng). If your 我 rises before 是, you'll hear something like "wo-o-shì": too much. 我 should be short and low, then 是 falls from the top.

The rules in one minute:
1. 3 + 3 → 2 + 3 (你好 "ní hǎo"). Not shown in pinyin.
2. 不 + tone 4 → bú (不是 bú shì). Shown in textbook pinyin.
3. 一 + tone 4 → yí; 一 + other tones → yì; number/ordinal/final → yī. Shown in textbook pinyin.
4. Non-final tone 3 → low (21), no rise.

Common pitfalls:

• ✗ Applying the 一 rule to numbers: 十一 is never "shíyí". ✓ shíyī.

• ✗ Forgetting that 个 is tone 4: 一个 = ✓ yí gè (even though 个 is often then said neutral: yí ge).

• ✗ Thinking the tone-3 rule only works inside a word: it applies across words too (我也 "wó yě", 你有 "ní yǒu").

• ✗ Applying 不 sandhi before a neutral syllable whose original tone was 4: it depends on the word → follow the audio.

How to practise? Pick 10 very frequent expressions containing these rules (你好, 很好, 可以, 不是, 不客气, 一个, 一起, 一下, 一点儿, 我也是) and repeat them until they come out without thinking. The rest will follow.