一旦…就…: once, as soon as (something happens)
复句 / complex sentences
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
一旦 ("one day when") introduces a decisive event, often negative or irreversible, with a consequence: "once", "if ever", "the moment that".
It can go before or after the subject. The second part has 就 or 便. Versus 一…就…: 一…就… = quick, habitual succession; 一旦 = decisive, sometimes hypothetical condition.
Examples
一旦失去信用,就很难再找到合作伙伴。
Yídàn shīqù xìnyòng, jiù hěn nán zài zhǎodào hézuò huǒbàn.Once you lose your credibility, it's very hard to find partners again.
经济一旦出现问题,普通人最先受到影响。
Jīngjì yídàn chūxiàn wèntí, pǔtōngrén zuì xiān shòudào yǐngxiǎng.The moment the economy runs into trouble, ordinary people are hit first.
习惯一旦养成,就不容易改变。
Xíguàn yídàn yǎngchéng, jiù bù róngyì gǎibiàn.Once a habit forms, it's hard to change.
Common mistakes
✗ 一旦 for a routine habit: ✗ 我一旦下班就回家 → ✓ 我一下班就回家。
✗ Dropping 就: ✓ 一旦…就….