HSK 5
一旦...就... (yídàn...jiù) in Chinese
一旦...就... says that once a condition or event occurs, a result follows or becomes difficult to reverse. 一旦 introduces the triggering point, and 就 commonly marks the consequence. The pattern often concerns future possibilities, rules, or decisive changes and can sound stronger than a neutral 如果 condition.
STRUCTURE
一旦 + Event, Subject + 就 + Result
When to use 一旦…就… — and when not
Use it when
- For a decisive future trigger: 一旦收到通知，我们就出发。
- To warn about a hard-to-reverse result: 一旦错过机会，就很难再找回来。
- For a general rule: 机器一旦过热，系统就会自动关闭。
Don't use it when
- For a freely repeated whenever relation; 每当...就... may fit repeated events better.
- Do not stack 一旦 and 就 before the condition.
- Do not require 就 in every context; the result may use 会, 必须, or another clear consequence marker.
Right vs. wrong
一旦就收到通知，我们出发。
一旦收到通知，我们就出发。
一旦 and 就 are stacked before the condition
一旦 introduces the condition 收到通知, while 就 belongs in the result clause before 出发.
一旦收到通知，就我们出发。
一旦收到通知，我们就出发。
就 is placed before the result-clause subject
When the result clause has the subject 我们, put it before the adverb 就.
Examples
一旦收到通知，我们就出发。
Yídàn shōudào tōngzhī, wǒmen jiù chūfā.
Once we receive the notice, we will leave.
一旦错过机会，就很难再找回来。
Yídàn cuòguò jīhuì, jiù hěn nán zài zhǎo huílái.
Once the opportunity is missed, it is hard to get it back.
机器一旦过热，系统就会自动关闭。
Jīqì yídàn guòrè, xìtǒng jiù huì zìdòng guānbì.
Once the machine overheats, the system shuts down automatically.
Common mistakes
一旦就收到通知，我们出发。
Yídàn jiù shōudào tōngzhī, wǒmen chūfā.
Move 就 to the result clause: 一旦收到通知，我们就出发。
一旦收到通知，就我们出发。
Yídàn shōudào tōngzhī, jiù wǒmen chūfā.
Put the subject before 就: 我们就出发。
Think you've got it?
Write your own 一旦…就… sentence and check that grammar point first.
一旦...就... (yídàn...jiù) FAQ
What is the difference between 一旦 and 如果?
如果 neutrally states a condition. 一旦 focuses on the moment the condition becomes real and often implies that the following result then follows quickly or is hard to reverse.
Does 一旦 always refer to the future?
No. It can describe general rules or past situations when context supplies the time. Its core meaning is the triggering point, not a fixed tense.