HSK 5 grammar · conjunction
一旦
一旦 — once / in case
Pattern
+ +
( ) can be left out · other words may come before, between and after
How it works
Sets up a hypothetical future condition whose consequence follows, usually with 就: 'once X happens, then Y.' It often carries a note of caution about an outcome that would be hard to reverse, unlike neutral 如果.
Examples
一旦做了决定，就不要轻易改变。
Yídàn zuò le juédìng, jiù bùyào qīngyì gǎibiàn.
Once you've made a decision, don't change it lightly.
一旦养成坏习惯，就很难改掉。
Yídàn yǎngchéng huài xíguàn, jiù hěn nán gǎidiào.
Once you form a bad habit, it's very hard to break.
Related
See it in a story
In the app, 一旦 is underlined wherever it appears in the stories you read, and practised with sentences from them. Free.