用于疑问句或感叹句中（表示意外、惊讶等，与how， why， who连用）。

例如：

How should I know?

Why should you be worrying?

Who should come to meet us but the manager!

Why was it that the manager would go back on his words?

2.用在某些状语从句中。例如：

He took a taxi to the station so that she should not miss the train.（也可根据需要，用might 或could）

They removed and buried all machines for fear (that) the enemy should make use of them.

3.用在I’m surprised， It worries me， It’s a pity， It’s unthinkable等结构后面的句子中表示惊讶、忧虑、惋惜、欢欣等情绪。例如：

I’m surprised that he should eat so little every day.

他每天吃得那么少，真使我感到惊讶。

It’s a pity that she should fail in the entrance examination.

她没有考取，真是可惜。

4.表示应该做，且有一种道义上的责任。例如：

I should help him because he is in trouble now.

You should do it because you have promised to.

比较：

would常用来解释为什么没做某事。例如：

I would go to the football match but the weather is too bad. So I won’t.

I would do it but I have something too urgent to be delayed.

5.意为“可能，该”，表示对现在情况、将来情况或过去情况的某种推测。例如：

He should arrive at noon.他该在中午到达。

Jim should be at home.吉姆可能在家。

He should be taking a bath now.他可能正在洗澡。

They should have finished the work by tomorrow.

到明天他们就可能完成工作了。

They should have reached the town by now.

他们现在该已经到达那座小城了。