一、名词性从句的基本概念
在复合句中起名词作用的从句叫名词性从句，包括作主语从句、表语从句、宾语从句和同位语从句。
(1)主语从句
就是在复合句中作主语的从句。主语从句常用that, whether, who, whom, whose, what, which, when, where, how, why, whatever, whoever等引导。如：
Whether he will be able to come remains a question. 他是否来还是问题。
That China is a great socialist country is well known. 众所周知，中国是一个伟大的社会主义国家。
注：为避免头重脚轻，使句子平衡，常用it作形式主语，而将主语从句后置。如上述第二例常说成：
It is well known that China is a great socialist country.
但是，由连接代词what, whatever, whoever等引导的主语从句一般不能用形式主语。如：
What he found surprised me greatly. 他的发现使我非常吃惊。
Whoever is finished may rest. 凡是做完工的人都可以休息。
(2)表语从句
就是在连系动词之后作表语的从句。引导表语从句的连接除与引导主语从句的连接词外，还有as if， as though。如：
The question is whether he is able to do it alone. 问题是他能否单独做这件事。
It looks as if (though) it is going to snow. 天好像要下雪似的。
注意：要区分以下句式：
1. that’s why+结果；that’s because+原因。2. the reason why /for…is that…
He is absent. That’s because he is ill. 他缺席，这是因为他生病了。
He is ill. That’s why he is absent. 他病了，这就是他缺席的原因。
The reason why he is absent is that he is ill. 他缺席的原因是他生病了。
(3)宾语从句
就是在复合句中作宾语的从句。引导这宾语从句的连接词与引导主语从句的连接词基本相同。如：
They didn’t say which they wanted. 他们没有说他们想要哪一个。
I am sure (that) no harm will ever come to you. 我肯定你永远不会受伤害。
I was surprised at what has happened.我对发生的事感到惊讶。
注意：当think, believe, suppose等的疑问式后面跟连接代词或连接副词引导的宾语从句时，习惯上将这些连接词置于句首，即“连接代/副词＋do you think /believe /expect＋宾语从句的其余部分？”如：
Who do you think is the best player this year? 你认为今年谁是最佳运动员?
What do you suppose you will do after school? 你想放学后干什么？
(4)同位语从句
就是在句中作同位语的从句。它通常有放在thought, idea, news, word(=news), plan, doubt, question, fact, suggestion, belief等抽象名词的后面，说明这些词的具体内容或含义。同位语从句大多由that引导，也可由whether, how, when, where, why等引导，但不能由which引导。如：
The idea that the earth is round is not a new one. “地球是圆的”这种观点并不新鲜。
He must answer the question whether he agrees to it or not. 他必须回答他是否同意此事这们一问题。
I have no idea when he will set out. 我不知他什么时候出发。
注：有时，同位语从句并不紧跟在它所说明的词的后面。如：
The whole truth came out at last that he was a wolf in sheep’s clothing. 真相终于大白了，他原来是一只披羊皮的狼。
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