一旦 (yīdàn) is used to express what would happen after a certain
condition is met. Usually it is used with 就. This grammar pattern
expresses that once one thing happens, something else will
happen.
Structure
This structure is similar to the English "once...then," as in
"once the company goes bankrupt, your family will lose all its
money."
一旦 + Condition，Subj. + 就 + Consequence
Examples
- 一旦 我 找 好 的 工作，我 就 会 安心。Once I find a good job, then I'll be able to
relax.
- 这 张 桌子 不 稳定，一旦 发生 什么 小 小 的 振动，就 会
倒塌。This table isn't very steady. Once there's
even a small vibration, it will fall over.
- 学习 计算机 科学 的 学生 很 自信，他们 一旦 毕业，就 会 找到 很 好 的
工作。Computer science students are very
confident. Once they graduate, they will be able to find good
jobs.
- 千万 不 要 在 经济 泡沫 中 炒股，一旦 市场 崩溃，你 会 很 后悔 的！You must absolutely not trade stocks in an economic bubble.
Once the market collapses, you will regret it!
Notice in the last example there is no 就. Sometimes you can
leave out the 就 and the context that the result will happen is
pretty clear.
Sources and further
reading
Books
Category:
B2 grammar points