HSK 2
The 一...就... Structure in Chinese
一...就... links two actions as "as soon as A, (then) B". 一 goes before the first verb and 就 before the second: 我一到家就睡觉 (as soon as I get home, I sleep). It shows B follows A immediately, and both 一 and 就 are required — dropping either breaks the pattern.
STRUCTURE
一 + Action A, 就 + immediate Action B
When to use 一…就 — and when not
Use it when
- One action triggers another right away: 他一下课就回家。
- A habit that always follows: 我一喝咖啡就睡不着。
- Two subjects, immediate reaction: 老师一说，学生就懂了。
Don't use it when
- Dropping 就 — both halves are required: 我一到家睡觉 is missing 就。
- For actions with a long gap — 一...就 means immediately; use 然后 for a loose sequence.
- Treating 一 as the number "one" — it takes no measure word: 一到, not 一个到。
Right vs. wrong
我一到家睡觉。
我一到家就睡觉。
Dropping 就
The pattern needs both halves: 一 before the first action and 就 before the second. Without 就, the second clause has no marker and the "as soon as" link breaks.
我一个到家就睡觉。
我一到家就睡觉。
Treating 一 as the number with a measure word
The 一 in 一...就 is not the number one — it directly precedes the verb (一到). Inserting a measure word like 个 turns it into "one" and breaks the structure.
Examples
我一到家就睡觉。
Wǒ yí dào jiā jiù shuìjiào.
As soon as I get home, I sleep.
他一下课就回家。
Tā yí xiàkè jiù huí jiā.
He goes home as soon as class ends.
我一喝咖啡就睡不着。
Wǒ yì hē kāfēi jiù shuì bu zháo.
As soon as I drink coffee, I can't sleep.
老师一说，学生就懂了。
Lǎoshī yì shuō, xuéshēng jiù dǒng le.
As soon as the teacher explained, the students understood.
她一看见我就笑了。
Tā yí kànjiàn wǒ jiù xiào le.
She smiled as soon as she saw me.
Common mistakes
我一到家睡觉。
Wǒ yí dào jiā shuìjiào.
Needs 就 before the second action: 我一到家就睡觉。
我一个到家就睡觉。
Wǒ yí gè dào jiā jiù shuìjiào.
一 here isn't "one"; no measure word: 我一到家就睡觉。
我到家一就睡觉。
Wǒ dào jiā yī jiù shuìjiào.
一 goes before the first verb, 就 before the second: 我一到家就睡觉。
Think you've got it?
Write your own 一…就 sentence and check that grammar point first.
The 一...就... Structure FAQ
What does 一...就... mean?
"As soon as A, then B" — B happens right after A. 一 goes before the first verb, 就 before the second: 我一起床就喝水 (as soon as I get up, I drink water).
Do I need both 一 and 就?
Yes. Both are required and each sits before its verb. Dropping 就 (我一到家睡觉) or misplacing them breaks the pattern.
Is the 一 in 一...就 the number "one"?
No. It's a structural marker meaning "as soon as", so it takes no measure word: 一到, 一看, 一听 — never 一个到.