便: then, right away (written, 一……便……)
【六09】【六44】副词、复句 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
便 is the written 就: it goes after the subject and before the verb and links two actions that follow each other at once. The commonest pattern is 一……便…… ("as soon as…", "no sooner… than…"): 他一下课便回家了.
Register: narration, press, careful prose. In speech, say 就: 他一下课就回家了 (see 一……就……). Its more formal cousin is 一经……便/就…… ("once…").
Even without 一, 便 marks an immediate follow-up: 抬头便看见了她. Do not confuse it with the 便 of 方便 or 随便, which is a different use.
Examples
他一下课便回家了。
Tā yí xià kè biàn huí jiā le.He went home as soon as class was over.
天一亮，我们便出发了。
Tiān yí liàng, wǒmen biàn chūfā le.As soon as it was light, we set off.
孩子们一见到老师，便围了上去。
Háizimen yí jiàndào lǎoshī, biàn wéi le shàngqu.The moment they saw their teacher, the children crowded round him.
她一毕业便决定回国工作。
Tā yí bìyè biàn juédìng huíguó gōngzuò.She decided to go back and work in her own country as soon as she graduated.
我一走出校门，抬头便看见了她。
Wǒ yì zǒuchū xiàomén, táitóu biàn kànjiàn le tā.I had hardly walked out of the school gate when I looked up and saw her.
他一听到母亲的声音，便再也忍不住了。
Tā yì tīngdào mǔqīn de shēngyīn, biàn zài yě rěn bu zhù le.The moment he heard his mother's voice, he could hold back no longer.
Common mistakes
✗ 便他回家了。 → ✓ 他便回家了。 (便 comes after the subject, never at the head of the sentence).
✗ 好的，我便去。 → ✓ 好的，我就去。 (in a chat 便 sounds stiff: say 就).