1. 在句子中起表语作用的从句叫做表语从句。表语从句的引导词和主语从句的引导词相同。

What the police want to know is when you entered the room.

警察想知道的是你什么时候进的房间。

The trouble is that we are short of funds. 困难是我们缺乏资金。

That's why I want you to work there. 那就是我要你在那儿工作的原因。

His first question was whether Mr. Smith had arrived yet.

他的第一个问题是史密斯先生到了没有。

as if, as though, because也可用来引导表语从句。

She seems as if she had done a great thing. 她看起来好像做了一件大事。

It is because you eat too much. 那是因为你吃得太多了。

2. 虚拟语气：表语从句

主语是idea, notion, proposal, suggestion, request等名词时, 作表语从句的动词为原形动词或should+原形动词。

My suggestion is that we (should) go and help him. 我的建议是我们应该去帮助他。

Our only request is that this should be settled as soon as possible.

我们唯一的请求就是尽快解决这个问题。