一 is written yī in dictionaries, but before another syllable its spoken tone often changes. The practical rule is:
- before a fourth tone, 一 becomes second tone: yí;
- before a first, second, or third tone, it becomes fourth tone: yì;
- by itself, at the end of a word or phrase, and in many number-reading contexts, it stays first tone: yī.
This is tone sandhi: a regular pronunciation change triggered by the surrounding tones. The meaning remains “one,” and the character does not change.
The three pronunciations of 一
| Environment | Pronunciation | Example |
|---|---|---|
| alone or phrase-final | yī | 一 yī — one |
| before Tone 4 | yí | 一个 yí ge — one (general measure word) |
| before Tone 1, 2, or 3 | yì | 一天 yì tiān — one day |
The change helps avoid an awkward sequence of pitch movements. Mandarin speakers learn it as part of ordinary word rhythm, not as a calculation performed during every sentence. The University of Iowa’s open pronunciation text summarizes the same 一 tone-sandhi pattern.
Before a fourth tone, yī becomes yí
A first-tone 一 changes to a rising second tone when the next syllable has a falling fourth tone:
- 一个 — yí ge — one; a
- 一样 — yíyàng — the same
- 一定 — yídìng — certainly; definitely
- 一半 — yíbàn — half
- 一共 — yígòng — altogether
Listen to the pair as one movement: rise into fall. Saying yī gè carefully is understandable, but yí ge is the normal connected-speech form.
Before Tones 1, 2, and 3, yī becomes yì
Before any of the other three full tones, 一 takes a falling fourth tone:
| Following tone | Example | Spoken form |
|---|---|---|
| Tone 1 | 一天 | yì tiān — one day |
| Tone 2 | 一年 | yì nián — one year |
| Tone 3 | 一起 | yìqǐ — together |
Other common examples include 一杯 (yì bēi, one cup), 一条 (yì tiáo, one long or flexible item), and 一点 (yìdiǎn, a little).
The spelling yī still tells you the word’s underlying dictionary tone. The surface pronunciation tells you what happens inside the phrase.
When does 一 stay yī?
一 normally keeps first tone when it is not directly modifying a following syllable in the usual sandhi relationship.
When it stands alone or comes last
我要一。
Wǒ yào yī.
I want one.
It also stays yī in expressions where it ends the unit being said, such as 二十一 (èrshíyī, twenty-one).
When reading digits, codes, dates, or ordered numbers
Phone numbers, room numbers, serial numbers, and years are commonly read digit by digit with yī:
2021 — èr líng èr yī
In mainland speech, the digit 1 may be read yāo in phone numbers or other strings where clarity matters. That is a separate numeral-reading convention, not the yí/yì sandhi rule.
In ordinal numbers with 第
In 第一 (dì-yī, first), 一 keeps first tone because it ends the ordinal expression. If another syllable follows as part of a larger phrase, speakers still treat 第一 as a stable unit: 第一个 is dì-yī ge, not dì-yí ge.
What happens in 一 + measure word phrases?
Measure words make the rule especially frequent:
- 一个人 — yí ge rén — one person
- 一本书 — yì běn shū — one book
- 一杯茶 — yì bēi chá — one cup of tea
- 一件事 — yí jiàn shì — one matter
Use the tone of the measure word, not the later noun. In 一个人, 个 is normally neutral in everyday speech but historically and pedagogically belongs to fourth tone; 一 is therefore pronounced yí.
Why do some pinyin sources still write yī?
Dictionaries normally list the base form yī. Textbooks differ: some mark the changed pronunciation to help beginners, while others keep the lexical tone and expect learners to apply sandhi.
Neither convention means the spoken rule is optional. When you see 一定 written yīdìng, a natural recording will still usually sound like yídìng. Treat written pinyin as notation, then trust clear native audio for the phrase’s actual shape.
How to make the change automatic
Do not drill three isolated versions of 一. Practise short groups:
- Rise before fall: yí ge, yí cì, yí bàn.
- Fall before the other tones: yì tiān, yì nián, yì běn.
- Keep first tone at the end: dì-yī, èrshíyī.
- Shadow complete phrases: copy timing and stress, not only pitch direction.
Our guide to tones in real Mandarin speech explains why these dictionary forms bend inside connected phrases. The related rule for 不 is simpler: it changes only before another fourth tone.
Once the combinations sound familiar, you will stop choosing among yī, yí, and yì. The following syllable will pull the right version out automatically.
