一旦

yīdàn

conjunction HSK 5 #1,028

Meanings

Literally "one dawn" - from the moment something happens.

一旦决定了，就不要后悔。

Yīdàn juédìng le, jiù búyào hòuhuǐ.

Once you've decided, don't regret it.

一旦发生火灾，请立即离开大楼。

Yīdàn fāshēng huǒzāi, qǐng lìjí líkāi dàlóu.

In case of fire, please leave the building immediately.

一旦你开始学习，就会发现中文很有趣。

Yīdàn nǐ kāishǐ xuéxí, jiù huì fāxiàn Zhōngwén hěn yǒuqù.

Once you start learning, you'll find Chinese is very interesting.

grammar

一旦…就… is a fixed pattern meaning "once… then…". The 就 part describes the consequence.

usage

一旦 often implies something irreversible or something with significant consequences. It's more dramatic than 如果 (if).

一 yī

旦 dàn