一 + measure + 就 + Verb
Use this when you want to highlight that an action happened almost instantly after a tiny trigger — 'the moment X, Y followed.' The 一 + measure word names the smallest possible unit of the trigger (one look, one sip, one moment), and 就 delivers the consequence with no delay. It is the Chinese workhorse for cause-and-effect that feels immediate and a little inevitable.
Structure
一 [MEASURE/QUANTITY] 就 [VERB]
yī [MEASURE] jiù [VERB]
How to Think About It
This is 一...就... in its quantity-emphasizing form. The measure word turns the trigger into something countably small — '一看就懂' means understanding came after exactly one look, no more needed. Drop the measure word and you get the plain 一...就... pattern (一到家就睡觉). Adding the measure word does one job: it dramatizes how little input was required for the result.
Examples
这道题我一看就会。
zhè dào tí wǒ yí kàn jiù huì.
I knew how to solve this problem the moment I saw it.
他一喝就醉。
tā yì hē jiù zuì.
He gets drunk after a single drink.
孩子一听见妈妈的声音就笑了。
háizi yì tīngjiàn māma de shēngyīn jiù xiào le.
The child smiled the moment he heard his mother's voice.
Common Mistake
Learners add 了 right after 就 because they associate completion with 了. But 就 already carries the sense of 'right then'; piling 了 in between 就 and the verb breaks the rhythm. The 了 belongs at the very end if anywhere, marking the completed event.
他一看就了懂。
他一看就懂了。
Don't Confuse With
一...就...
Same family. Use the plain version (一到家就睡觉) when the trigger is an event; use 一 + measure + 就 (一看就懂) when you want to count how few units of the trigger were needed.
才 + Verb
Opposite feel. 才 = 'only then, finally.' 就 = 'right away, easily.' 一看就懂 means understood instantly; 看了三遍才懂 means understood only after the third look.
立刻 / 马上
These are time adverbs you add to a single clause. 一...就... links TWO clauses — the trigger and the response.
Practice
Fill the blank: 这个字我____看就认识。
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一
Fill the blank: 她一尝____知道是谁做的。
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就
Arrange: 就 / 一 / 我 / 累 / 跑
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我一跑就累。
Translate to Chinese: He understood the moment he heard it.
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他一听就懂。
Write a sentence describing a fast reaction using 一 + measure + 就.
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我一看天气就知道要下雨了。
Hear It in Real Episodes
This pattern appears in 1 Fluentide episode: