一旦……就……
Reach for 一旦...就... when one event will trigger another the moment it happens — 'once X, then Y immediately.' It's used for conditions whose outcome is automatic or irreversible: once you click send, once the money's transferred, once the contract is signed. Stronger than 如果...就... because it stresses that the trigger event sets things in motion you can't take back. Common in warnings, contracts, and news commentary.
Structure
一旦 [TRIGGER], 就 [CONSEQUENCE]
yīdàn... jiù...
How to Think About It
一旦 is a one-way door. The 就 clause isn't just a result — it's the automatic, irreversible flip that follows the trigger. Use 如果 for ordinary 'if'; switch to 一旦 only when crossing that line changes things permanently or when the result is dramatic. Saying 一旦下雨我就回家 sounds melodramatic; saying 一旦发出去, 信息就改不了了 sounds right.
Examples
一旦做了决定, 就不要后悔。
Yīdàn zuò le juédìng, jiù bùyào hòuhuǐ.
Once you've made the decision, don't regret it.
一旦你离开公司, 就拿不到这笔钱了。
Yīdàn nǐ líkāi gōngsī, jiù ná bù dào zhè bǐ qián le.
Once you leave the company, you won't get this money.
一旦发出去, 信息就改不了了。
Yīdàn fā chūqù, xìnxī jiù gǎi bù liǎo le.
Once the message is sent, you can't change it.
Common Mistake
Learners use 一旦 for any 'if' statement, including small everyday choices. 一旦 carries weight — keep it for triggers that lead to serious or unstoppable outcomes.
一旦我饿, 我就吃饭。
我饿了就吃饭。
Don't Confuse With
如果...就...
如果 is neutral 'if.' 一旦 emphasizes that the trigger is a point of no return. Use 如果 for trivial conditions, 一旦 for heavy ones.
只要...就...
只要 = 'as long as,' a sufficient condition that can keep applying. 一旦 = 'the moment it happens,' a one-time trigger.
一...就...
一...就... means 'as soon as' (sequence of events) and works for any pair of actions. 一旦 specifically frames the first event as a serious trigger.
Practice
____做了决定, 就不要后悔。
Show answer
一旦
一旦发出去, 信息____改不了了。
Show answer
就
Arrange: 一旦 / 钱 / 就会 / 消失 / 转过去 / 她 / 了
Show answer
一旦钱转过去了, 她就会消失。
Once the money is transferred, this girl will disappear.
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一旦钱转过去了, 这个女孩子就会消失。
Write a sentence with 一旦...就... about something that can't be undone.
Show answer
一旦养了小狗, 你就要每天照顾它。
Hear It in Real Episodes
This pattern appears in 1 Fluentide episode: