名词性从句是在句子中取名字词用途的主谓结构，它可以在主从复合句中作主语、宾语、表语、介词的宾语与同位语。
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：从句可以坐落于句首，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？
一般在口语或非正式文体中，that常可省略，但需要是在用先行词it的状况下，如：
Its a pity （that） you are leaving.
⑵由wh-疑问词引导：从句既能够前置，又可以借用于先行词it后置。如：
Who cleaned the meeting room is unknown. Its 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可以坐落于句首，如：
Is it known1 where she went？
⑶主语从句也可由名词性关系词引导，这种从句只能坐落于句首，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，这个时候that仅起连接用途，不充当从句中的成分。假如连接词要在从句中起语法用途，就要用连接词what等来引导从句。
I believe that he is honest. We fully2 understood what he meant.
⑵有的形容词的后面也可跟宾语从句。这种形容词有：certain，afraid，aware，sorry，sure，worried等。如：
Im 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，此时即便主句是一个疑问句，宾语从句也需要维持陈述句的语序。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可以由that来引出，只限于except，in，but，besides，save等五个介词的宾语从句，其它介词后面不可以跟由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. Im thinking of whom I should ask for help.
3. 表语从句
表语从句一般由that引出，当然how，why，where，when等也可以引出表语从句。容易见到的表语从句有下面两种句型：
⑴The reasonthat：表示缘由是。如：
The reason for leaving was that it was too cold.
⑵It （This，That） is because：表示这是由于，这是因为原故。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质内容。
⑴同位语从句起进一步讲解说明先行词有哪些用途，一般由that引导。同位语从句的先行词一般是一些抽象名词，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 fortunewhat others sought after with zestwas all rubbish to him.
⑷注意同位语和that引导的定语从句有什么区别：that在同位语从句中只起引导从句有哪些用途，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。）
