一、名詞性從句的基本概念
在復合句中起名詞作用的從句叫名詞性從句，包括作主語從句、表語從句、賓語從句和同位語從句。
(1)主語從句
就是在復合句中作主語的從句。主語從句常用that, whether, who, whom, whose, what, which, when, where, how, why, whatever, whoever等引導。如：
Whether he will be able to come remains a question. 他是否來還是問題。
That China is a great socialist country is well known. 眾所周知，中國是一個偉大的社會主義國家。
注：為避免頭重腳輕，使句子平衡，常用it作形式主語，而將主語從句后置。如上述第二例常說成：
It is well known that China is a great socialist country.
但是，由連接代詞what, whatever, whoever等引導的主語從句一般不能用形式主語。如：
What he found surprised me greatly. 他的發現使我非常吃驚。
Whoever is finished may rest. 凡是做完工的人都可以休息。
(2)表語從句
就是在連系動詞之后作表語的從句。引導表語從句的連接除與引導主語從句的連接詞外，還有as if， as though。如：
The question is whether he is able to do it alone. 問題是他能否單獨做這件事。
It looks as if (though) it is going to snow. 天好像要下雪似的。
注意：要區分以下句式：
1. that’s why+結果;that’s because+原因。2. the reason why /for…is that…
He is absent. That’s because he is ill. 他缺席，這是因為他生病了。
He is ill. That’s why he is absent. 他病了，這就是他缺席的原因。
The reason why he is absent is that he is ill. 他缺席的原因是他生病了。
(3)賓語從句
就是在復合句中作賓語的從句。引導這賓語從句的連接詞與引導主語從句的連接詞基本相同。如：
They didn’t say which they wanted. 他們沒有說他們想要哪一個。
I am sure (that) no harm will ever come to you. 我肯定你永遠不會受傷害。
I was surprised at what has happened.我對發生的事感到驚訝。
注意：當think, believe, suppose等的疑問式后面跟連接代詞或連接副詞引導的賓語從句時，習慣上將這些連接詞置于句首，即“連接代/副詞+do you think /believe /expect+賓語從句的其余部分?”如：
Who do you think is the best player this year? 你認為今年誰是最佳運動員?
What do you suppose you will do after school? 你想放學后干什么?
(4)同位語從句
就是在句中作同位語的從句。它通常有放在thought, idea, news, word(=news), plan, doubt, question, fact, suggestion, belief等抽象名詞的后面，說明這些詞的具體內容或含義。同位語從句大多由that引導，也可由whether, how, when, where, why等引導，但不能由which引導。如：
The idea that the earth is round is not a new one. “地球是圓的”這種觀點并不新鮮。
He must answer the question whether he agrees to it or not. 他必須回答他是否同意此事這們一問題。
I have no idea when he will set out. 我不知他什么時候出發。
注：有時，同位語從句并不緊跟在它所說明的詞的后面。如：
The whole truth came out at last that he was a wolf in sheep’s clothing. 真相終于大白了，他原來是一只披羊皮的狼。
