As soon as: 一……就
固定格式 / fixed patterns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
(Subject 1) + 一 + V1，(Subject 2) + 就 + V2
一……就 means the second action follows immediately after the first: "as soon as…". It can also express a habit: every time A happens, B follows.
一 and 就 are adverbs: they go after their subject, before the verb. 他一回家就睡觉了 / 我一说，他就明白了.
Pronunciation: 一 changes tone depending on what follows (yì / yí); you'll often hear yí before a 4th tone: 一到 (yí dào).
Examples
水一开，就把面条放进去。
Shuǐ yì kāi, jiù bǎ miàntiáo fàng jinqu.As soon as the water boils, put the noodles in.
我一闻到这个味道就饿了。
Wǒ yì wéndào zhège wèidao jiù è le.As soon as I smell this, I get hungry.
他一学就会，真聪明！
Tā yì xué jiù huì, zhēn cōngming!He picks things up instantly, so clever!
Common mistakes
✗ 一他回家就睡觉。 → after the subject. ✓ 他一回家就睡觉。
✗ 我一到家，就我吃饭。 → 就 follows the second subject. ✓ 我一到家就吃饭。