名词性从句是在句子中起名词作用的主谓结构，它可以在主从复合句中作主语、宾语、表语、介词的宾语以及同位语。

　　1. 主语从句

　　主语从句可以由连词that，wh-疑问词或名词性关系词引导。

　　⑴由连词that引导：从句可以位于句首，但常常用形式主语it替代，从句本身则放在主句后面，如：
　　That he will come to the party is certain. It happened that the harvest was bad that year. Is it true that she has never been there？
　　通常在口语或非正式文体中，that常可省略，但必须是在用先行词it的情况下，如：
　　It‘s a pity （that） you are leaving.

　　⑵由wh-疑问词引导：从句既可以前置，又可以借助于先行词it后置。如：
　　Who cleaned the meeting room is unknown. It‘s still a question when we shall have our sports meet. It is uncertain whether he will come.注意：在疑问句中，wh-疑问词所引导的从句不能位于句首，如：
　　Is it known¹ where she went？

　　⑶主语从句也可由名词性关系词引导，这类从句只能位于句首，如：
　　Where she went was Los Angeles. What will be，will be. Whomever you invite will be welcome.

　　2. 宾语从句

　　⑴宾语从句可用that引导，这时that仅起连接作用，不充当从句中的成分。如果连接词要在从句中起语法作用，就要使用连接词what等来引导从句。
　　I believe that he is honest. We fully² understood what he meant.

　　⑵有些形容词的后面也可跟宾语从句。这类形容词有：certain，afraid，aware，sorry，sure，worried等。如：
　　I‘m afraid that you are wrong on this point. I am aware that you have been exceptionally³ kind to me.

　　⑶宾语从句也可以用how，when，who，which，where，why等词引出，此时即使主句是一个疑问句，宾语从句也必须保持陈述句的语序。如：
　　Go and ask why he was late yesterday. Can you tell me where the hospital is？

　　⑷跟在介词后面的宾语从句一般不能由that来引出，只限于except，in，but，besides，save等五个介词的宾语从句，其它介词后面不能跟由that引出的宾语从句，而只能跟由what，how，whether等引出的宾语从句。
　　You are wrong in that you took effect for cause. I would go with you，except that I have to work that day. They were arguing about what was to be done. I‘m thinking of whom I should ask for help.

　　3. 表语从句

　　表语从句通常由that引出，当然how，why，where，when等也可以引出表语从句。常见的表语从句有下面两种句型：

　　⑴“The reason…that”：表示“原因是……”。如：
　　The reason for leaving was that it was too cold.

　　⑵“It （This，That） is because…”：表示“这是因为……”，“这是由于……缘故”。如：
　　This is because iron⁴ contains more carbon⁵ than steel.

　　4. 同位语从句

　　同位语从句表示与之同位的名词中心词的实际内容。

　　⑴同位语从句起进一步解释说明先行词的作用，通常由that引导。同位语从句的先行词通常是一些抽象名词，如：appeal，idea，truth，fact，theory，belief，plan，hope，proposal，rumor，suggestion等。
　　There is truth in the old saying that necessity⁶ is the mother of invention.

　　⑵由wh-疑问词引导，如：
　　I have no idea when he will come. It is difficult to answer your question why I did it.

　　⑶由名词性关系词引导，如：
　　Fame and fortune—what others sought after with zest—was all rubbish to him.

　　⑷注意同位语和that引导的定语从句的区别：that在同位语从句中只起引导从句的作用，本身并无意义，也不在句子中担当任何成分；而定语从句的that既引导从句又充当句子成分。
　　The news that he intended to come gave us much pleasure.（同位语从句，that不作句子分。）
　　He is the best speaker that addressed the meeting.（定语从句，that在从句中作宾语，指代的先行词是主句中的主语。）