在句子中起名词作用的句子叫名词从句。名词从句的功能相当于名词词组, 它在复合句中能担任主语、宾语、表语、同位语、介词宾语等，因此根据它在句中不同的语法功能，名词从句又可分别称为主语从句、宾语从句、表语从句和同位语从句。
17.1 引导名词性从句的连接词
引导名词性从句的连接词可分为三类：
连接词： that, whether ,if (不充当从句的任何成分)
连接代词：what, whatever, who, whoever, whom, whose, which
连接副词：when, where, how, why
不可省略的连词：
1. 介词后的连词
2. 引导主语从句和同位语从句的连词不可省略。例如：
That she was chosen made us very happy. 她被选上了，我们很高兴。
We heard the news that our team had won. 我们听到消息说我们队赢了。
比较：whether与if 均为\"是否\"的意思。 但在下列情况下，whether 不能被if 取代：
1. whether引导主语从句并在句首
2. 引导表语从句
3. whether从句作介词宾语
4. 从句后有\"or not\"
大部分连接词引导的主语从句都可以置于句末，用 it充当形式主语。例如：
It is not important who will go. 谁去，这不重要。
It is still unknown which team will win the match. 到底谁赢呢，形势尚不明朗。
17.2 名词性that-从句
1)由从属连词that引导的从句叫做名词性that-从句。That只起连接主句和从句的作用，在从句中不担任任何成分，本身也没有词义。名词性that-从句在句中能充当主语、宾语、表语、同位语和形容词宾语。例如：
主语：That he is still alive is sheer luck. 他还活着全靠运气。
宾语：John said that he was leaving for London on Wednesday. 约翰说他星期三要到伦敦去。
表语：The fact is that he has not been seen recently. 事实是近来谁也没有见过他。
同位语：The fact that he has not been seen recently disturbs everyone in his office.
近来谁也没有见过他，这一情况令办公室所有的人不安。
形容词宾语：I am glad that you are satisfied with your job. 你对工作满意我感到很高兴。
2)That-从句作主语通常用it作先行词，而将that-从句置于句末。例如：
It is quite clear that the whole project is doomed to failure. 很清楚，整个计划注定要失败。
It\'s a pity that you should have to leave. 你非走不可真是件憾事。
用it作形式主语的that-从句有以下四种不同的搭配关系：
a. It + be +形容词+ that-从句
It is necessary that… 有必要……
It is important that… 重要的是……
It is obvious that… 很明显……
b. It + be + -ed 分词+ that-从句
It is believed that… 人们相信……
It is known to all that… 从所周知……
It has been decided that… 已决定……
c. It + be +名词+ that-从句
It is common knowledge that… ……是常识
It is a surprise that… 令人惊奇的是……
It is a fact that… 事实是……
d. It +不及物动词+ that-分句
It appears that… 似乎……
It happens that… 碰巧……
It occurred to me that… 我突然想起……