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