万一…
in case; if by any chance (something unwelcome)
Structure
万一 可能性很小的坏事 ， 应对
What it means
In case; if by any chance: raises an unlikely possibility, usually an unwelcome one, so you can guard against it.
How it is built
万一 + unlikely event，(就) + response: 万一下雨，我们就在家看电影 (If it happens to rain, we will watch a film at home). As a tag: 带上伞吧，万一下雨呢？
When to use it
One chance in ten thousand: unlikely, but worth preparing for. 如果 is the neutral 'if'. As a noun: 以防万一, 'just in case'. Spoken and written.
Watch out
Only for unlikely events; certain or routine ones take 如果 or nothing: ✗ 万一冬天来了，就穿厚衣服 ✓ 冬天来了，就穿厚衣服.
Examples
万一遇到自称“保本高收益”的产品，一定要小心。
If you should meet a product claiming guaranteed principal and high returns, be careful.
带上伞吧，万一下雨呢？
Take an umbrella. What if it rains?
Learn this in Lesson 57 · HSK 6: 存钱、炒股与理财 (Saving, stocks and money management)
A dialogue, a reading, word cards with audio and a drill that uses it, in Learn Chinese with Bao. The first lessons of every level are free.
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