HSK 4
万一... (wànyī): In Case in Chinese
万一 introduces a contingency the speaker considers possible but usually unlikely, unwanted, or worth preparing for. The following clause gives a response or consequence and may use 就, 那, or another result expression. It is stronger than a neutral 如果 because it adds the speaker's concern about what might unexpectedly happen.
STRUCTURE
万一 + Condition, Subject + 就 + Response
When to use 万一…（就）… — and when not
Use it when
- To prepare for a possible problem: 万一下雨，我们就改在室内见。
- To ask about an unwanted possibility: 万一赶不上最后一班车怎么办？
- To give a precautionary reason: 带上充电器，万一手机没电呢？
Don't use it when
- For a neutral or routinely expected condition; 如果 is usually more suitable.
- Do not place 万一 after the contingency predicate it is meant to introduce.
- Do not require 就 in every sentence; questions with 怎么办 and other responses can follow directly.
Right vs. wrong
明天下雨万一，我们就改时间。
万一明天下雨，我们就改时间。
万一 is placed after the contingency clause
万一 introduces the possible event, so it must come before 明天下雨 rather than after the event.
万一明天下雨，就我们改在室内见。
万一明天下雨，我们就改在室内见。
就 is placed before the result-clause subject
In the result clause, put the subject 我们 before the adverb 就: 我们 + 就 + 改.
Examples
万一下雨，我们就改在室内见。
Wànyī xiàyǔ, wǒmen jiù gǎi zài shìnèi jiàn.
If it happens to rain, we will meet indoors instead.
万一赶不上最后一班车怎么办？
Wànyī gǎn bu shàng zuìhòu yì bān chē zěnme bàn?
What will we do if we miss the last bus?
带上充电器，万一手机没电呢？
Dài shàng chōngdiànqì, wànyī shǒujī méi diàn ne?
Bring a charger, in case the phone runs out of power.
Common mistakes
明天下雨万一，我们就改时间。
Míngtiān xiàyǔ wànyī, wǒmen jiù gǎi shíjiān.
Move 万一 before the contingency: 万一明天下雨。
万一明天下雨，就我们改在室内见。
Wànyī míngtiān xiàyǔ, jiù wǒmen gǎi zài shìnèi jiàn.
Put the result-clause subject before 就: 我们就改在室内见。
Think you've got it?
Write your own 万一…（就）… sentence and check that grammar point first.
万一... (wànyī): In Case FAQ
What is the difference between 万一 and 如果?
如果 presents a condition neutrally. 万一 highlights a possibility that is unexpected, undesirable, or important to prepare for, so it often carries concern.
Does a 万一 sentence need 就?
No. 就 commonly introduces the response, but 万一 can also be followed by 怎么办, 呢, or another result expression when the relationship is clear.