一旦…就…
once …; the moment … (the result follows)
Structure
一旦 条件 ， 就 结果
What it means
Once something happens, a serious result follows. It marks a turning point whose outcome is weighty and often hard to undo.
How it is built
一旦 + condition，(subject) + 就 + result: 一旦养成坏习惯，就很难改了 (Once a bad habit forms, it is hard to change). 一旦 can also follow the subject: 人一旦失去健康….
When to use it
Neutral to formal; for big, often hypothetical turning points, and frequent in warnings. The light everyday version is 一…就… for quick sequences: 我一到家就睡觉.
Watch out
Not for trivial routines: ✗ 我一旦起床就刷牙 ✓ 我一起床就刷牙. 一旦 suits weighty results: 一旦失去信任，就很难找回.
Examples
一旦失去平衡，人就会生病。
Once the balance is lost, illness follows.
一旦养成坏习惯，就很难改了。
Once a bad habit forms it is hard to change.
Learn this in Lesson 33 · HSK 5–6: 中医与养生 (Chinese medicine and keeping well)
A dialogue, a reading, word cards with audio and a drill that uses it, in Learn Chinese with Bao. The first lessons of every level are free.
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