解:表语从句.
理由:表句从句就是"表句是个句子",同理,宾语从句,定语从句都这样理解.
表语从句放在连系动词之后,充当复合句中的表语/
下面是我找到的材料,参考一下吧.
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基本概念
1． 定义：用作表语的从句叫做表语从句。
2． 构成：关联词+简单句
3． 引导表语从句的关联词的种类：
(1) 从属连词that。如：
The trouble is that I have lost his address. 麻烦是我把他的地址丢了。
(2) 从属连词whether, as, as if。如：
He looked just as he had looked ten years before. 他看起来还与十年前一样。
The question is whether they will be able to help us. 问题是他们是否能帮我们。
注：从属连词if一般不用来引导表语从句，但as if却可引导表语从句，如：
All this was over twenty years ago, but it´s as if it was only yesterday.
这都是20多年前的事了，但宛如昨天一样。
能跟表语从句的谓语动词一般为系动词be, seem, look等。如：
It looked as if it was going to rain. 看起来天要下雨了。
(3)连接代词who, whom, whose, what, which, whoever, whatever, whichever
连接副词 where, when, how, why。
如：The problem is who we can get to replace her. 问题是我们能找到谁去替换她呢。
The question is how he did it. 问题是他是如何做此事的。
That was what she did this morning on reaching the attic. 那就是她今晨上了阁楼干的。
重点疑难
1． 连词because可引导表语从句。如：
I think it is because you are doing too much. 我想这是因为你做得太多。
2． 在一些表示“建议、劝说、命令”的名词后面的表语从句中，谓语动词用虚拟语气。should+动词原形表示，should可省略。如：
My suggestion is that we (should) start early tomorrow. 我的建议是我们明天一早就出发。
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