Expressing "what if" with "wanyi"

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万一 (wànyī)

Expressing "Just in Case" in Statements

表示可能性极小的假设，多用于说话人不希望发生的情况。后面的分句一般是应对这种不如意情况的具体方法。“万一”可以用在主语前，也可以用在主语后。

万一⋯⋯ ，⋯⋯。

万一他不喜欢，你可以来换。 Wànyītā bù xǐhuan, nǐ kěyǐ lái huàn. In case he doesn't like it, you can come to make an exchange.
我需要记下来，万一忘了，可以查。Wǒ xūyào jì xiàlái, wànyī wàng le, kěyǐ chá.I need to write it down. In case I forget, I can look it up here.
万一发生这种情况，你们就这样做。 Wànyī fāshēng zhè zhǒng qíngkuàng，nǐmen jiù zhèyàng zuò. In case this situation happens, do it this way.
万一有危险，马上给我打电话。 Wànyī yǒu wēixiǎn, mǎshàng gěi wǒ dǎ diànhuà.In case there is any danger, call me immediately.
万一迷路了，就用地图导航。 Wànyī mílù le, jiù yòng dìtú dǎoháng. In case you get lost, use the map navigation.

表示可能性极小的假设，多用于不如意的事情。“万一”可以用在主语前，也可以用在主语后。

万一⋯⋯ 怎么办？

or

万一⋯⋯ 呢？

万一我父母不同意呢？Wànyī wǒ fùmǔ bù tóngyì ne? What if my parents disagree?
他万一不在呢？你还是先打电话问问吧。 Tā wànyī bù zài ne? Nǐ háishì xiān dǎ diànhuà wèn wèn ba. What if he's not there? You'd better make a call first.
万一被老板发现了怎么办？ Wànyī bèi lǎobǎn fāxiàn le zěnme bàn? What if you got busted by the boss?
早点走吧，万一堵车呢？Zǎo diǎn zǒu ba, wànyī dǔchē ne?We should leave earlier. What if there is a traffic jam?
你喝了这么多酒还要开车？万一撞到人怎么办？Nǐ hē le zhème duō jiǔ hái yào kāichē? Wànyī zhuàngdào rén zěnme bàn? You want to drive after you drank so much? What if you hit someone?

Notice that 万一 (wànyī) is used as a noun here. 指可能性极小的意外变化，多是不好的。

北京的冬天特别冷，多带几件衣服 以防万一。 Běijīng de dōngtiān tèbié lěng, duō dài jǐ jiàn yīfu yǐ fáng wànyī.It's very cold in winter in Beijing. You should bring a few more pieces of clothing. Just in case.
这么晚出去，最好找一个男生陪你一起，以防万一。 Zhème wǎn chūqù, zuìhǎo zhǎo yī gè nánshēng péi nǐ yīqǐ, yǐ fáng wànyī .You'd better ask a boy to go out with you in such a late night. Just in case.
这些可能不够，多买一点吧，以防万一。 Zhèxiē kěnéng bù gòu, duō mǎi yīdiǎn ba, yǐ fáng wànyī.These are probably not good enough. We should buy some more. Just in case.