1. 在句子中起表語作用的從句叫做表語從句。表語從句的引導詞和主語從句的引導詞相同。

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

警察想知道的是你什么時候進的房間。

The trouble is that we are short of funds. 困難是我們缺乏資金。

That’s why I want you to work there. 那就是我要你在那兒工作的原因。

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

他的第一個問題是史密斯先生到了沒有。

as if, as though, because也可用來引導表語從句。

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

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

2. 虛擬語氣：表語從句

主語是idea, notion, proposal, suggestion, request等名詞時, 作表語從句的動詞為原形動詞或should+原形動詞。

My suggestion is that we (should) go and help him. 我的建議是我們應該去幫助他。

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

我們唯一的請求就是盡快解決這個問題。