for fear of/that 表示“由于害怕；惟恐,以免”of后接动名词,that后接从句用虚拟语气
Put on a coat for fear of catching a cold.穿上外衣以免感冒.
The weather in London is so changable that people always bring their umbrella with them for fear of rain.伦敦的天气很多变,人们总是要随身带着雨伞以防下雨.
He told us to keep quiet for fear that we might disturb others.
他让我们保持安静,以免打扰别人.
in case of 假使,万一
In case of fire ring the bell.万一起火,请按铃.
两者的区别：
In case of rain ,they won't go .假使下雨,他们就不去了.
Take an umbrella with you for fear of rain .带上一把伞,以免下雨.
in case 指“假如某种情况出现,我们要……”,for fear指“我们要……以免某种情况出现”,前者强调主语的能动性,不过,两者的区别好像不是很明显,