一旦…就【yídàn…jiù】

一旦 sets up a condition – something that may or may not happen, or that changes things for good once it does – and the second clause, usually with 就, gives the consequence: "once", "if ever", "in the event that".

一旦 can go before or after the subject. 就 goes after the subject of the second clause.

The consequence is often serious or hard to undo, and the sentence reads as a warning or an instruction.

一旦…就 or 一…就

一…就 is "as soon as": one event follows straight after another, often as a habit. 一旦 is about a condition, so it doesn't fit an everyday routine.

• × 我一旦回家就吃饭 → 我一回家就吃饭