Usage note
万一用来表示一种很小的可能性，提醒我们做好准备。
Wànyī yònglái biǎoshì yī zhǒng hěn xiǎo de kěnéngxìng, tíxǐng wǒmen zuò hǎo zhǔnbèi.
In case is used to indicate a small possibility, reminding us to be prepared.
Context note
我们常说‘万一……就……’，比如‘万一下雨，就带伞’。
Wǒmen cháng shuō ‘wànyī……jiù……’, bǐrú ‘wànyī xiàyǔ, jiù dài sǎn’.
We often say 'in case... then...', for example 'in case it rains, bring an umbrella'.
5 examples
万一下雨，我们就待在家里。
wàn yī xià yǔ， wǒ men jiù dài zài jiā lǐ。
In case it rains, we will stay at home.
带上水壶，万一渴了可以喝。
dài shàng shuǐ hú， wàn yī kě le kě yǐ hē。
Bring a water bottle in case you get thirsty.
万一迷路，就打电话给妈妈。
wàn yī mí lù， jiù dǎ diàn huà gěi mā ma。
In case you get lost, call mom.
多带一件衣服，万一天气变冷。
duō dài yí jiàn yī fu， wàn yī tiān qì biàn lěng。
Bring an extra coat in case the weather gets cold.
万一忘记作业，可以问同学。
wàn yī wàng jì zuò yè， kě yǐ wèn tóng xué。
In case you forget the homework, you can ask a classmate.
5 mini stories
小丽去公园玩。她带了伞，万一下雨。
xiǎo lì qù gōng yuán wán。 tā dài le sǎn， wàn yī xià yǔ。
Xiao Li goes to the park. She brings an umbrella in case it rains.
小明去远足。他多带了水，万一不够。
xiǎo míng qù yuǎn zú。 tā duō dài le shuǐ， wàn yī bú gòu。
Xiao Ming goes hiking. He brings extra water in case it's not enough.
小红去朋友家。她带了零食，万一饿了。
xiǎo hóng qù péng yǒu jiā。 tā dài le líng shí， wàn yī è le。
Xiao Hong goes to a friend's house. She brings snacks in case she gets hungry.
小华去图书馆。他带了笔和纸，万一要记东西。
xiǎo huá qù tú shū guǎn。 tā dài le bǐ hé zhǐ， wàn yī yào jì dōng xī。
Xiao Hua goes to the library. He brings a pen and paper in case he needs to write something.
小美去游泳。她带了创可贴，万一受伤。
xiǎo měi qù yóu yǒng。 tā dài le chuàng kě tiē， wàn yī shòu shāng。
Xiao Mei goes swimming. She brings band-aids in case she gets hurt.
Mini dialogue
带伞
A
今天可能下雨，你带伞了吗？
Jīntiān kěnéng xià yǔ, nǐ dài sǎn le ma?
It might rain today, did you bring an umbrella?
B
带了，万一真的下雨呢。
Dài le, wàn yī zhēn de xià yǔ ne.
Yes, in case it really rains.
多带水
A
我们多带一瓶水吧。
Wǒmen duō dài yī píng shuǐ ba.
Let's bring an extra bottle of water.
B
好的，万一路上渴了。
Hǎo de, wàn yī lùshang kě le.
Okay, in case we get thirsty on the way.
Related words and contrasts
similar
如果
rú guǒ
if
used for conditions, like 'in case'
related
准备
zhǔn bèi
to prepare
what you do 'in case' something happens
related
可能
kě néng
maybe
expresses possibility, like 'in case'