The boy hid himself behind the tree in case/for fear that his father should see him.

Take your raincoat in case/lest it should rain.

Take care lest you（should） fall. 当心不要跌倒。（也可用Take care in order not to fall或Take care so as not to fall代替。）

注意lest多用于书面语中，文体较古老些。for fear that比较通俗。口语中常用in case代替lest。它们各自带的目的状语从句中一般都用should表示虚拟语气，并且这个should往往还可省略。

意思都是“以防，以免”。

lest的从句一般要用虚拟语气，形式是“should+动词原形”或省掉should。

for fear that和in case从句一般用虚拟语气，但有时也可以用陈述语气。

Lest和For fear that等同义结构的用法

一、连词lest引出目的状语从句，谓语中包含should（体现虚拟语气），并且还可省略。

1. Please remind me of it again tomorrow lest I (should)forget. 明天请再提醒我一下，免得我忘记。

2. He runs fast lest he (should)miss the train. 他快跑，免得赶不上火车。

二、可用for fear that 和in case等来代替lest：

1. He locked up the document for fear that it (should) be stolen. （也可用lest代之。）他把文件锁好了，免得被窃。

2. I don’t make a noise in case I (should) wake them. 我不出声，免得惊醒他们。

3. Take care so as not to fall. 当心不要跌到。（也可用Take care in order not to fall和Take care lest you fall代之。）

三、lest用于fear，afraid，danger等词后接一个从句：

1. I am afraid lest (=that) he should get here too late. 我怕他到达这里太晚。

2. By the end of 1956 she had waited for you ten years in sore fear lest you had died. 到1956年底，她已经等了你十年之久。生怕你已死去。

in case+主语+动词可跟在陈述句或命令句后面：lest除在较正式的书面英语中出现外，一般不常用。

I obeyed her lest she should be angry. 我得顺着她，免得她生气。

1. for fear that +  might, could etc-subjunctive 由于害怕；惟恐，以免

He told us to keep quiet for fear that we might disturb others.

2. in case + different tenses  以防万一

I’ ll make a cake in case someone drops in at the weekend．

I carry a spare wheel in case I have／ should have a puncture．

I always keep candles in the house in case there is a power cut．

I always kept candles in the house in case there was a power cut．

若果，假使

In case I’m late, start without me.

3. lest + be-subjunctive 免得，以免, 惟恐

He doesn’ t／ didn’ t dare to leave the house lest someone ( should ) recognize him．

She pulled away from the window lest anyone ( should ) see them.

He paused, afraid lest he say too much.
1. for fear that +  might, could etc-subjunctive 由于害怕；惟恐，以免

He told us to keep quiet for fear that we might disturb others. 

2. in case + different tenses  以防万一

I’ ll make a cake in case someone drops in at the weekend．
I carry a spare wheel in case I have／ should have a puncture．
I always keep candles in the house in case there is a power cut．
I always kept candles in the house in case there was a power cut．

若果，假使
In case I’m late, start without me.

you'd better keep a seat for her in case she______
A. coming   B. should come    C. is coming  d. may come

she is working hard for fear that she _______ to pass the exam.
A. should fail  B. may fail   C. fails   D. would fail

the mad man was put in the soft-padded cell lest he ______ himself
A. injure  B. injured  C. had injured   D. would injure