Once…, then…: 一旦…就…
Marks a turning point — something that might happen, or has happened — after which a lasting, often serious consequence follows. Not a general “if”, and not for a one-off past event.
结构
（主语）+ 一旦 + 变化／事件，（主语）+ 就 + 持续的结果
讲解
本文中的中文词语都可以激活，查看词典释义。
， “Once a habit has formed, it is very hard to change.” picks out a moment of change and says that crossing it has consequences that stay. The first clause usually contains a verb of change or completion — often with — and the second usually carries and a serious, durable result.
The event after is typically one that has not happened yet and might or might not: ， “Once a phone falls into water, it is hard to repair.” It can also describe a general truth, as above, or a firm rule: ， “Once you have made a decision, don’t change it.”
Two boundaries matter. is not an everyday “if”: for an ordinary condition with no turning point — tomorrow’s weather, a plan for the weekend — use or . And it does not report a single past sequence of events; “as soon as we met last month we became friends” is ……: .
正确
Say each one out loud before moving on. The audio is a reference pronunciation, not a model to imitate exactly — match the tone contour, not the voice.
一旦养成了习惯，就很难改了。一旦養成了習慣，就很難改了。
Yídàn (second tone, rising) yǎngchéngle (third tone, dipping) xíguàn, (second tone, rising) jiù (fourth tone, falling) hěn (third tone, dipping) nán (second tone, rising) gǎi (third tone, dipping) le. (neutral tone)···
Once a habit has formed, it’s very hard to change.
手机一旦掉进水里，就很难修好了。手機一旦掉進水裡，就很難修好了。
Shǒujī (third tone, dipping) yídàn (second tone, rising) diàojìn (fourth tone, falling) shuǐ (third tone, dipping) li, (neutral tone) jiù (fourth tone, falling) hěn (third tone, dipping) nán (second tone, rising) xiūhǎo (first tone, high and level) le. (neutral tone)···
Once a phone falls into water, it’s very hard to repair.
你一旦做了决定，就不要再变了。你一旦做了決定，就不要再變了。
Nǐ (third tone, dipping) yídàn (second tone, rising) zuòle (fourth tone, falling) juédìng, (second tone, rising) jiù (fourth tone, falling) bú (second tone, rising) yào (fourth tone, falling) zài (fourth tone, falling) biàn (fourth tone, falling) le. (neutral tone)···
Once you’ve made a decision, don’t change it again.
错误 — 常见错误
These sentences are wrong. They are printed here on purpose, each one paired with the repair and the reason, because the error you can recognise is the error you stop making.
不太对
Incorrect sentence: 一旦明天下雨，我们就不去了。
正确
Correct sentence: 如果明天下雨，我们就不去了。
为什么: Tomorrow’s weather is an ordinary condition, not a turning point with lasting effects. Use 如果.
不太对
Incorrect sentence: 一旦养成了习惯，很难改了。
正确
Correct sentence: 一旦养成了习惯，就很难改了。
为什么: The consequence clause normally carries 就 after the subject; without it the two clauses lose their link.
不太对
Incorrect sentence: 我们上个月一旦见了面，就成了好朋友。
正确
Correct sentence: 我们上个月一见面就成了好朋友。
为什么: A single past sequence (“as soon as… then…”) takes 一…就…. 一旦 frames a possible or general turning point.
对比
Most errors with this pattern are not errors about this pattern — they are a neighbouring pattern arriving at the wrong moment. Read these side by side.
如果 sets up any condition at all, trivial or serious: 如果明天下雨，我们就不去了. 一旦 is narrower and heavier. It implies that the event is a threshold — something that, once crossed, changes the situation for good — and the result clause usually describes something hard to undo. 如果你骗了他 “if you lied to him” is neutral; 你一旦骗了他，他就不会再相信你了 “once you lie to him, he will never trust you again” stresses the point of no return.
If: 如果…（的话），就… →小测验
第 1 题 / 3总分: 0
Fill the gap: ___ 失去了大家的信任，就很难再找回来。