一…就…
As soon as…; whenever…
HSK 3HSK 3.0 · 3
(S1) + 一 + V1，(S2) + 就 + V2
一…就… has two meanings: (1) the second action happens right after the first (我一到家就给你打电话 'I'll call you as soon as I get home'); (2) whenever the first condition occurs, the second follows (他一喝酒脸就红 'his face goes red whenever he drinks'). Both 一 and 就 are adverbs, so they come after the subject and before the verb; with two subjects: 我一说，他就明白了 (not *就他明白了).
HSK 3–4 sentence ordering and gap fills.
Examples
我一到家就给你打电话。
I'll call you as soon as I get home.
他一喝酒脸就红。
His face turns red whenever he drinks.
老师一说，我就明白了。
As soon as the teacher explained, I understood.
天一黑，他就回家了。
He went home as soon as it got dark.
Exercises
1.我一下课 去食堂吃饭。
2.Choose the correct sentence:
3.我 看见他就高兴。
4.这个问题很简单，他一听就 了。