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。
Thats 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。