先说in case用法
一、用作连词+从句
in case 用作连词时有以下两种用法。
1. 表示条件，意为“如果”“万一”。如：
In case it rains， do not expect me. 如果下雨，就不要等我了。
In case you need something，please don't hesitate to let me know.如果你需要什么东西，请不客气地对我说。
In case you see him， ask him about it. 如果你见着他，问问他这事。
In case I forget， please remind me of my promise. 如果我忘了，请提醒我我的诺言。
In case he arrives before I get back， please ask him to wait. 如果他在我回来之前到，请让他等等我。
In case you think I was incautious，take a look at the map.
如果你认为我不够仔细，就自己看看地图。
Dad brought a notebook along to the beach，in case he was seized by sudden inspiration.
爸爸带了一个笔记本去海滩，如果突然有了灵感可以记录下来。
1.assume 假设 Assume 是指你主观而且没有事实根据所做的推测，也就是没有任何证据的假设。例： I can only assume that it was a mistake. 我只能假定那是个错误。例： We can’t just assume her guilt. 我们不。
In case you should need any help，here's my number.
万一你需要帮助的话，这是我的电话号码。
4、implicit assumption 含蓄的假定
2. 表示目的，意为“以防”“生怕”。它不是表示正面的目的，而是表示否定的目的；如：
Write the telephone number down in case you forget.把电话号码写下来以免忘了。
I'll stay around in case you need me.
我就待在这儿，也许你用得着我。
Take some spare clothes in case you get wet.
多带几件衣服，以备身上弄湿了好穿。
She said nothing in case her temper slipped out of gear.
她什么都没说，免得按捺不住情绪。
I was going to get out of the building in case it went up
suppose ：常用词，意义较广泛，指缺乏确切事实，根据一些现象进行的推测，也可指为论证而提出合乎逻辑推理的某种假定，有时仅表示自己的意见。I had supposed his wife a younger woman.我原以为他妻子要更年轻。Why do yo。
我怕大楼起火，正打算要跑出去。
Leave your key with a neighbour in case you lock yourself out one day.
给邻居留把钥匙，以防有朝一日把自己反锁在外面。
He left the door ajar in case I needed him.
他虚掩房门，以防我会需要他。
I'll stay around in case you need me.
我就待在这儿，也许你用得着我。
Extra boiling water should be kept at hand just in case it is needed.
身边应多准备一些开水，以备不时之需。
In case anyone was following me，I made an elaborate detour
为了防止有人跟踪我，我特地绕了弯路。
We are scared to abandon ourselves to our feelings in case we seem weak or out of control.
我们不敢放纵自己的情感，以免显得太过软弱或缺乏自制。
I’m shy of buying shares in case I lose money. 我不敢买股票，怕赔钱。
Take warm clothes in case the weather is cold. 带些暖和的衣服，以防天气变冷。
I’ll keep a seat for you in case you should change your mind. 我给你留一个座位，以防你会改变主意。
We took our swimming things in case we happened to find a pool. 我们带上游泳用品，以备万一能找到一个水池。
I always slept by the phone in case he should ring during the night. 我总是睡在电话机旁边，怕万一他夜间打电话来。
Keep quiet in case you（should）interrupt him when he is busy．
要保持安静， 以防打扰他。
She is now studying for fear that she（should）fail in English．
她现在很勤奋，生怕英语不及格。
You must wake him early lest he（should）be late for school．
你务必早点叫醒他，以免他上学迟到。
凡是由lest（以免；免得），in case（以防）和 for fear that（生怕，唯恐）引导的目的状语从句也有这个用法。
Better take an umbrella in case. 最好带把伞，以防下雨。
二、用于 in case of用法
in case of 为介词，其用法与用作连词时的二种用法相似。
1.表示条件，意为“如果”“万一”。如：
In case of rain，they can't go.
万一下雨，他们就不能去了。
In case of fire， ring the bell. 如发生火灾，就按此铃。
In case of emergency， break the glass and press the button. 遇到紧急情况时，击碎玻璃罩并按下按钮。
In case of my not being here， ask my brother to help you. 如果我不在，可让我弟弟帮助你。
The doctor asked us to call him during the night except in case of necessity.医生吩咐我们，除非必须，否则不要在夜里叫他。
2.表示目的，意为“以防”。如：
假设的英语是hypothesis
It would be safer to take more money with you in case of emergency.
多带点钱保险些，以防急用。
Security forces have been put on standby in case of violence.
安全部队已处于待命状态，以防出现暴力事件。
Many shops along the route have been boarded up in case of trouble.
沿途很多商店的门窗都用木板封上了，以防不测。
I keep an umbrella here in case of rain. 我在这里放一把雨伞以防下雨。
The wall was built along the river in case of floods. 沿江筑了堤坝，以防涨水。
补充说明：在某些特定的语境中，in case 有时有比较特殊的意思。如：
In no case will they look on passively.
他们决不会在一边看热闹。
In no case can I lose this important match.
无论如何我不能输掉这次重要比赛。
综上所述：下面这个句子，如果没有语境，可以有两种译法：
Do this in case a fire breaks out.
1.万一发生了火灾，你就这么做。
2.你要这么做，以免发生火灾。
