5 · Expressions (level 5)
In case and once: wànyī and yídàn
unwelcome and decisive conditions
Wànyī prepares for what you hope will not happen; yídàn names the point after which things change.
English “if” covers every condition. Chinese has a neutral 如果, and next to it two words with a colour of their own. 万一, literally “one in ten thousand”, introduces a possibility that is unlikely and unwelcome: 万一下雨，我们就在家看电影, “if by any chance it rains, we’ll stay in and watch a film”.
万一 can stand before or after the subject, and it is also a noun, “a contingency”: 以防万一, “just in case”. A question built on it expresses worry: 万一他不来怎么办？, “what if he doesn’t come?”.
一旦, literally “one morning”, marks a threshold: once something happens, everything after is different. The second clause usually carries 就: 一旦决定了，就不要后悔, “once you’ve decided, don’t look back”. It can describe the future, a general truth, or the past.
The two are easy to tell apart by asking whether the speaker wants the condition. 万一 is always something to guard against; using it in an invitation would sound as if you hoped the other person said no.
The form
| in case, unlikely | 万一下雨，我们就在家看电影。 |
| just in case | 带把伞吧，以防万一。 |
| what if …? | 万一他不来怎么办？ |
| once, decisively | 一旦决定了，就不要后悔。 |
| once, after the subject | 习惯一旦养成，就很难改。 |
The rules one by one
The form this topic is aboutThe word that calls for it
万一 introduces a possibility that is unlikely and usually unwelcome.
万一他不在家，你就给我打电话。If by any chance he isn’t at home, give me a ring.
万一 can also be a noun: a contingency.
多带点儿钱，防个万一。Take a bit of extra money, just in case.
一旦 marks a decisive change; what follows is the consequence, usually with 就.
一旦失去信任，就很难再找回来。Once trust is lost, it is very hard to win back.
一旦 can stand before or after the subject, and can describe a general truth as well as the future.
他一旦开始工作，就什么都忘了。Once he starts working, he forgets everything else.
For a plain, neutral condition use 如果, not 万一.
如果明天天气好，我们去爬山。If the weather is good tomorrow, we’ll go hiking.
A closer look
Three kinds of “if”
如果 is neutral and carries no feeling. 万一 is a small risk you would rather avoid — it is what people say when they pack a first-aid kit. 一旦 is a line which, once crossed, changes everything after it; that is why it is so common in warnings and rules.
The literal meanings are a good memory aid. 万一 is “one chance in ten thousand”, so it is unlikely. 一旦 is “one morning, one day”: the day it happens, things are different. Note how the tone of 一 changes: wànyī at the end of a word keeps the first tone, yídàn before a fourth tone takes the second.
How to spot it
- 万一
- 以防万一
- 一旦…就
- 万一…怎么办
Example sentences
万一飞机晚点怎么办？What if the plane is delayed?
你把我的电话号码存好，万一有事可以找我。Save my number, so you can reach me if anything comes up.
一旦发现问题，要马上报告。As soon as you find a problem, report it straight away.
这种病一旦严重起来，就很难治。Once this illness gets serious, it is very hard to treat.
万一我忘了，你提醒我一下。In case I forget, give me a reminder.
机会一旦错过，就不会再来了。Once an opportunity is missed, it won’t come again.
7 more example sentences
多准备一份材料，万一不够呢。Prepare an extra copy of the documents, in case there aren’t enough.
晚上别一个人去，万一出事怎么办？Don’t go alone at night — what if something happens?
我给你一把钥匙，万一我不在，你可以自己开门。I’ll give you a key, so that if I’m not in you can let yourself in.
一旦签了合同，就不能随便改了。Once the contract is signed, it can’t be changed at will.
孩子一旦爱上读书，父母就不用操心了。Once children come to love reading, their parents needn’t worry any more.
密码一旦忘了，就得重新设置。Once you forget the password, you have to set a new one.
你还是买个保险吧，以防万一。You’d better take out insurance, just in case.
Worth knowing (1)
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Wànyī … ne: and what if …?
A clause with 万一 and a final 呢 is a complete sentence: it voices a worry and leaves the consequence unsaid. The listener is expected to supply it, or to reassure.
万一他不同意呢？And what if he doesn’t agree?
The trap English speakers fall into
Watch out
English “if you happen to be free” softens a friendly invitation, and learners reach for 万一 to say “happen to”. It cannot do that job: it casts the condition as an unlikely accident to guard against. In an invitation it sounds as if you were hoping the other person would be busy. A neutral “if” is 如果 or 要是.
Wànyī prepares for what you hope will not happen; yídàn names the point after which things change.
Your turn
8 exercises on this topic. Your answers are checked in the browser; none of them leaves your device.