名词性从句
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.
主语从句
用作主语的从句叫主语从句。
(That the earth goes around the sun) is known to all.
(How this happened) is not clear to anyone.
(Whoever comes) is welcome.
(When we shall have our sports meeting) is still a question.
(Whether he will come today) is still unknown.
注意：
1. 主语从句是特殊疑问句，应该用特殊疑问词加陈述句语序。
2. that引导的主语从句放在句首，不可省略that。
3. 主语从句常用it作形式主语，真正的主语放在后面，这时that可省略。
It's a pity (that) you missed such a good opportunity.
It's still a question when we shall buy the new equipment.
It's still unknown whether he will come or not.
4. It is + P.P.+ that ; It + Vi + that，It作形式主语。
It is said that the president will visit our company.
It seems that he has caught a cold.
