The number yī (一, meaning "one") is everywhere in Mandarin. It appears in counting, dates, phone numbers, and dozens of common phrases. Its original tone is the 1st Tone, high and level. But depending on what follows it, 'yī' changes its tone in two different ways.
The Two Rules
Rule 1: Before a 4th Tone syllable, 'yī' changes to the 2nd Tone (rising).
Rule 2: Before a 1st, 2nd, or 3rd Tone syllable, 'yī' changes to the 4th Tone (falling).
In other words, 'yī' almost never keeps its original 1st Tone in connected speech. It adapts to whatever comes next.
When 'Yī' Does Keep the 1st Tone
The original 1st Tone survives only when 'yī' is used alone, at the end of a phrase, or when counting in sequence (yī, èr, sān…). In these cases, there is no following syllable to trigger a change.
Examples: Before a 4th Tone (yī → 2nd Tone)
- yī dìng (Definitely) → spoken as yí dìng
- yī yàng (The same) → spoken as yí yàng
- yī jiàn (One piece) → spoken as yí jiàn
Examples: Before 1st, 2nd, or 3rd Tones (yī → 4th Tone)
- yī tiān (One day) → spoken as yì tiān, 'tiān' is 1st Tone
- yī nián (One year) → spoken as yì nián, 'nián' is 2nd Tone
- yī qǐ (Together) → spoken as yì qǐ, 'qǐ' is 3rd Tone
How to Remember It
Think of 'yī' as a word that avoids clashing with its neighbor. Before a falling 4th Tone, it rises. Before everything else, it falls. The logic is the same as 'bù': avoid two similar contours sitting side by side.
A quick reference:
- Before 4th Tone → 'yī' becomes 2nd Tone (rising)
- Before 1st, 2nd, or 3rd Tone → 'yī' becomes 4th Tone (falling)
- Alone or at the end → 'yī' stays 1st Tone (high level)
When yī Keeps Its First Tone
The base first tone survives in more cases than just counting. Whenever yī acts as a label or an ordinal rather than a quantity, it stays high and level. You hear the original tone in dì-yī (第一, first), in a date like yī yuè (January, the first month), and in a sequence such as yī, èr, sān. The tone changes only when yī means a real count of one and a syllable follows it, so spotting whether it is a number or a label tells you which tone to use.
Practice
Read these and decide the tone of 'yī' before speaking:
- yī bēi (One cup): 'bēi' is 1st Tone → say yì bēi
- yī kuài (One piece/dollar): 'kuài' is 4th Tone → say yí kuài
Check the tone of the next syllable first. The answer follows automatically.
