Level 4 · Upper Intermediate
Once (something happens): 一旦…就
一旦 A，（Subj.）就 B
一旦 (yídàn) means "once (this happens)" — often with a warning tone: 一旦开始，就不能停 "once you start, you can't stop". The event hasn't happened yet; 一旦 imagines the moment it does, and 就 delivers the consequence.
It's stronger and more written than the everyday 一…就. Mind the difference in time, too: 一…就 happily describes real past sequences (我一到家就给你打电话了), while 一旦 stays hypothetical and future-facing.
Examples
一旦开始，就不能停。
一旦開始，就不能停。
yī dàn kāi shǐ jiù bù néng tíng
Once you start, you can't stop.
一旦你决定了，就不要再改变。
一旦你決定了，就不要再改變。
yī dàn nǐ jué dìng le jiù bù yào zài gǎi biàn
Once you've decided, don't change it again.
这样的机会一旦错过，就不会再有了。
這樣的機會一旦錯過，就不會再有了。
zhè yàng de jī huì yī dàn cuò guò jiù bù huì zài yǒu le
Once a chance like this is missed, it won't come again.
一旦下雪，路就不好走了。
yī dàn xià xuě lù jiù bù hǎo zǒu le
Once it snows, the roads become hard to walk on.
Common mistakes
✗ 我一旦到家，就给你打电话了。
✓ 我一到家，就给你打电话了。
Real past sequences take 一…就 — 一旦 imagines a future "once".
Related grammar points
Practice this pattern in graded stories like Hard to Stop (Part 1) inside the Literate Chinese app.
