HSK 5
一旦⋯⋯，就⋯⋯
yí dàn ⋯ ⋯ ， jiù ⋯ ⋯
Once..., then...
Explanation
一旦⋯⋯，就⋯⋯ (yídàn..., jiù...) is a pair of conjunctions (a pair of words connecting the clauses of a sentence) forming a hypothetical compound sentence (a sentence with two clauses stating a condition that may occur). It means "once... then...": as soon as the condition occurs, the result follows immediately.
一旦 (yídàn) + [a condition/event that has not yet happened but is possible]，就 (jiù) + [result]. 一旿 goes at the head of the condition clause and can come before or after the subject (中文一旦学起来 - the subject before 一旦 is also fine); 就 goes at the head of the result clause.
When hypothesizing something in the future such that, once it has occurred, the result is almost inevitable, emphasizing the certainty and speed of the consequence (一旦考试不及格，就要延期毕业 - the moment you fail the exam, you must postpone graduation).
一旦 (yídàn) stresses "once it has happened, then...," usually for something not yet occurred but possible; different from 一...就... (yī...jiù... as soon as... then...) which is used for two actions following each other immediately and usually already or certainly occurring.
Vietnamese learners often forget the 就 (jiù) clause in the result, or use 一旦 for something that is certain to happen. 一旦 is for HYPOTHETICAL situations; and you should have 就 opening the following clause to complete the pair.
Examples
Once I fail the exam, I will have to postpone graduation.
Once the subway is completed, traffic jams can be greatly relieved.
Once you start learning Chinese, you can never put it down again.
Practice
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Learn “一旦⋯⋯，就⋯⋯” effectively
The grammar point “一旦⋯⋯，就⋯⋯” (Once..., then...) is one of the key structures at HSK 5. To use it fluently, you need to understand the contexts where it's used and practice regularly.
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