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
一旦发生火灾，请立刻离开大楼。
Yídàn fāshēng huǒzāi, qǐng lìkè líkāi dàlóu.
Once a fire breaks out, please leave the building immediately.
一旦失去信任，就很难再建立起来。
Yídàn shīqù xìnrèn, jiù hěn nán zài jiànlì qǐlái.
Once trust is lost, it's hard to rebuild.
一旦决定了，他就不会改变。
Yídàn juédìng le, tā jiù bù huì gǎibiàn.
Once he decides something, he won't change his mind.
一旦数据泄露，公司就要承担法律责任。
Yídàn shùjù xièlòu, gōngsī jiù yào chéngdān fǎlǜ zérèn.
Once data is leaked, the company has to bear legal responsibility.
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.
一旦房价开始下跌，就很难再回到原来的水平。
Yídàn fángjià kāishǐ xiàdiē, jiù hěn nán zài huídào yuánlái de shuǐpíng.
Once housing prices start to fall, it's hard for them to return to their previous level.
一旦泄露出去，就再也收不回来了。
Yídàn xièlòu chūqù, jiù zài yě shōu bù huílái le.
Once it leaks out, you can't take it back.
一旦决定了，就别再犹豫。
Yídàn juédìng le, jiù bié zài yóuyù.
Once you've decided, stop hesitating.
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...
Neutral 'if...then.' No weight, no urgency. Use for everyday hypotheticals. 一旦 adds inevitability and usually negative stakes.
只要...就...
'As long as...' — frames the trigger as a sufficient condition, often positive ('as long as you try, you'll succeed'). 一旦 frames it as a dangerous threshold.
一...就...
'As soon as...' — immediate sequence ('as soon as I got home, I slept'). About timing, not about a triggered consequence.
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.
Practice
Fill in: ____你答应了，____必须做到。
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一旦, 就
Translate to Chinese: Once the contract is signed, it cannot be changed.
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一旦签了合同，就不能改了。
Arrange: 一旦 / 就 / 习惯 / 养成 / 改 / 很难
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习惯一旦养成，就很难改。
Use 一旦...就... to warn about a consequence in business or daily life.
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一旦错过这次机会，就没有下一次了。
____做了决定, 就不要后悔。
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一旦
一旦发出去, 信息____改不了了。
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就
Arrange: 一旦 / 钱 / 就会 / 消失 / 转过去 / 她 / 了
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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.
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一旦养了小狗, 你就要每天照顾它。
___ 涨起来，就有更多人想买。
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一旦
Hear It in Real Episodes
This pattern appears in 6 Fluentide episodes. Press play to hear the sentence:
鲸鱼一旦死亡，体内的细菌就开始分解蛋白质和脂肪。
Once a whale dies, the bacteria inside its body start breaking down proteins and fats.
Sperm Whale Carcass Closes Four Sydney BeachesAdvanced
但是一旦有一个人做到了，魔咒就破了。
But once one person breaks it, the curse breaks.
Sebastian Sabbe Runs Official 1:59:30 MarathonAdvanced
可是，一旦钱转过去了，这个女孩子就会消失。
But, once the money was transferred, this girl would disappear.
AI Romance Scams Use Fake Video CallsIntermediate
一旦涨起来，就有更多人想买。
Once it starts going up, more people want to buy.
KOSPI Hits 7,500 as Korean Stocks SurgeIntermediate
Related Grammar Patterns
Acquire by listening
Hear 一旦……就…… in real Chinese, not in a textbook.
Fluentide picks the next news episode at your level, so this pattern shows up again and again in real context. The transcript marks it, the audio drills it. Free to start.