# Chinese Tone Sandhi Rules: 3rd Tone, 一 (yī) and 不 (bù) Explained

Source: https://sentencecoach.com/grammar/tone-sandhi
Level: HSK 1–3

In spoken Mandarin, certain tones automatically change depending on the syllable that follows (变调 tone sandhi). Two third tones in a row turn the first into a second tone (你好 nǐ hǎo → ní hǎo). 不 (bù) becomes second tone bú before another fourth tone (不是 bú shì). 一 (yī) stays first tone when counting, becomes second tone yí before a fourth tone (一个 yí gè), and becomes fourth tone yì before tones 1, 2 and 3 (一天 yì tiān, 一起 yìqǐ).

Patterns:
- 3rd tone + 3rd tone → 2nd tone + 3rd tone: 你好 (ní hǎo), 很好 (hén hǎo), 可以 (kéyǐ).
- 不 (bù) + 4th tone → bú + 4th tone: 不是 (bú shì), 不去 (bú qù), 不客气 (bú kèqi).
- 一 (yī) + 4th tone → yí + 4th tone: 一个 (yí gè), 一下 (yí xià), 一定 (yídìng).
- 一 (yī) + 1st / 2nd / 3rd tone → yì: 一天 (yì tiān), 一年 (yì nián), 一起 (yìqǐ).
- 一 (yī) when counting, in ordinals or at the end → yī: 第一 (dì-yī), 一二三 (yī èr sān), 统一 (tǒngyī).

Examples:
- 你好，请问洗手间在哪儿？ (nǐ hǎo, qǐng wèn xǐ shǒu jiān zài nǎr?) — Hello, excuse me, where is the restroom?
- 我今天不去图书馆，我想在家看一本书。 (wǒ jīn tiān bú qù tú shū guǎn, wǒ xiǎng zài jiā kàn yì běn shū.) — I'm not going to the library today; I want to read a book at home.
- 我们一起去买一杯咖啡吧。 (wǒ men yì qǐ qù mǎi yì bēi kā fēi ba.) — Let's go buy a cup of coffee together.
- 这是我第一次来北京。 (zhè shì wǒ dì yī cì lái běi jīng.) — This is my first time coming to Beijing.

Common mistakes:
- ✗ Pronouncing 不去 as bù qù (two falling tones) → ✓ Pronounce 不去 as bú qù (rising + falling): Before a fourth-tone syllable, 不 always rises to second tone (bú).
- ✗ Pronouncing 第一 as dì-yí or dì-yì → ✓ Pronounce 第一 as dì-yī (level first tone): In ordinals (第一), phone numbers and at the end of a word, 一 keeps its original first tone yī.

FAQ:
- Q: Should I write tone sandhi marks in pinyin?
  A: Standard textbooks and dictionaries write base tones (bù, yī, nǐ hǎo), though the official 2025 HSK syllabus marks sandhi on lexicalised words like 不客气 (búkèqi) and 一起 (yìqǐ). Paste any sentence into our Pinyin & Tone Sandhi Converter to see both side by side.
- Q: How do three third tones in a row change?
  A: They follow word groups: 展览馆 (zhǎnlǎn + guǎn) becomes zhán lán guǎn, whereas 我很好 (wǒ + hěn hǎo) becomes wǒ hén hǎo.