在句子中起名词作用的句子叫名词性从句(Noun Clauses)。名词性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,名词性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。

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
That she was chosen made us very happy.
We heard the news that our team had won.

【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",例:Whether he will come or not is not clear.

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:

It is not important who will go.
It is still unknown which team will win the match.