How to use 万一
万一 + possible event，response; 万一 + event + 怎么办 / 呢？
万一 raises a contingency the speaker wants to allow for, often something unlikely or undesirable. The point is preparedness or concern rather than a numerical estimate of probability. It can introduce a question about what would happen or explain why a precaution is sensible. The following response may show how the problem would be handled.
Use 怎么办 to ask what to do if the event occurs, or 呢 to leave the concern as a conversational what if. A full conditional sentence can instead give the planned response. The expression 以防万一 describes taking precautions just in case. Context determines the actual likelihood, so do not read 万一 as a literal statistical claim.
Examples with pinyin and English
万一我父母不同意呢？
What if my parents disapprove?
万一被发现了怎么办？
What if we get caught?
带着伞吧， 万一下雨呢？
Take an umbrella. What if it rains?
你先打电话问问。他万一不在呢？
You should call first. What if he's not there?
What to remember
- Use 万一 to raise a contingency.
- Ask 怎么办 when the response is uncertain.
- Recognize 以防万一 as a precautionary expression.
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Source and editorial notes
Explanation rewritten for Laowai. Examples are adapted from the Laowai English grammar curriculum, with editorial corrections where needed. The original curriculum credits AllSet Learning’s Chinese Grammar Wiki as used with permission. Read the retained source credits.
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