不然就...
Reach for this when you want to spell out a fallback or threat — 'otherwise,' 'or else.' It introduces what happens if the preceding course of action ISN'T taken: do X, or else Y. Common in warnings ('hand it in by Friday — otherwise you fail'), suggestions ('let's eat now, otherwise I'll faint'), and rules ('register first, otherwise it's illegal'). The 就 in this pattern locks the alternative as the immediate, automatic consequence.
Structure
[ACTION OR CONDITION]，不然就 [ALTERNATIVE / CONSEQUENCE]
bùrán jiù...
How to Think About It
不然 literally means 'not so,' i.e., 'if not this.' Adding 就 ('then immediately') tightens it into 'otherwise, this kicks in.' The pattern always points BACKWARD — it depends on a prior statement to be the 'this.' You can't start a fresh paragraph with 不然就 and expect listeners to follow; they need to know what the alternative replaces.
Examples
你快走吧，不然就赶不上车了。
Nǐ kuài zǒu ba, bùrán jiù gǎn bú shàng chē le.
You'd better go now, otherwise you'll miss the bus.
记得交税，不然就违法了。
Jìde jiāo shuì, bùrán jiù wéifǎ le.
Remember to pay taxes, otherwise it's illegal.
我们得先订餐厅，不然就没位子了。
Wǒmen děi xiān dìng cāntīng, bùrán jiù méi wèizi le.
We should book the restaurant first, otherwise there won't be any seats.
Common Mistake
Learners use 不然 standalone where the consequence is short, dropping 就. Without 就, the consequence feels detached from the warning. Also, learners sometimes confuse 不然 with 然后 ('then / next'), but those are different — 然后 is sequential, 不然 is conditional.
你要快点，不然迟到。
你要快点，不然就迟到了。
Don't Confuse With
否则
否则 = 'otherwise,' more formal, used in writing and instructions. 不然就 is the conversational sibling. Same meaning, different register.
要不然
要不然 = colloquial 'or else / or how about.' Often used to suggest an alternative course rather than warn ('要不然我们明天去 — or how about we go tomorrow'). 不然就 leans more toward consequence.
要不就
要不就 = 'or just' — proposes a fallback option. Similar but lighter than 不然就. 要不就 sounds like 'fine, let's just X'; 不然就 sounds like 'otherwise X happens.'
Practice
你要交税，___ 违法。
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不然就
我们得带伞，不然 ___ 淋雨了。
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就
Arrange: 你 / 不然就 / 早点 / 迟到了 / 睡
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你早点睡，不然就迟到了。
Hurry up, otherwise the movie will already have started.
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快点，不然就电影已经开始了。
Use 不然就 to spell out a consequence if someone doesn't act.
Show answer
你得多喝水，不然就头疼了。
Hear It in Real Episodes
This pattern appears in 1 Fluentide episode: