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。

一. 主语从句
  主语从句是在复合句中充当主语的从句,通常放在主句谓语动词之前或由形式主语it代替,而本身放在句子末尾。

1. It 作形式主语和it引导强调句的比较
It 作形式主语代替主语从句,主要是为了平衡句子结构,主语从句的连接词没有变化。而it引导的强调句则是对句子某一部分进行强调,无论强调的是什么成分,都可用连词that。被强调部分指人时也可用who/whom。例如:
    a) It is a pity that you didn’t go to see the film.  你不去看那场电影真可惜。
    b) It doesn’t interest me whether you succeed or not. 我对你成功与否不感兴趣。
    c) It is in the morning that the murder took place. 谋杀案是在早上发生的。(强调句型)
    d) It is John that broke the window. 是John打碎的窗户。(强调句型)

2. 用it 作形式主语的结构
(1) It is + 名词 + 从句
          It is a fact that …                     事实是…
          It is an honor that                      …非常荣幸
          It is common knowledge that                 …是常识
(2) It is + 形容词 + 从句
          It is natural that…                        很自然…
          It is strange that…                        奇怪的是…
(3) It is + 不及物动词 + 从句
          It seems that…                          似乎…
          It happened that…                      碰巧…
          It appears that…                         似乎…
(4) It + 过去分词 + 从句
          It is reported that…                    据报道…
          It has been proved that…               已证实…
          It is said that…                          据说…

3. 主语从句不可位于句首的五种情况:
(1)if 引导的主语从句不可居于复合句句首。
(2)It is said /reported…结构中的主语从句不可提前。例如:
        正确表达:It is said that President Jiang will visit our school next week.
        错误表达:That President Jiang will visit our school next week is said. 
(3)It happens/occurs…结构中的主语从句不可提前。例如:
       正确表达:It occurred to him that he failed in the examination.
       错误表达:That he failed in the examination occurred to him.
(4)It doesn’t matter how/whether …结构中的主语从句不可提前。例如:
       正确表达:It doesn’t matter whether he is wrong or not.
       错误表达:Whether he is wrong or not doesn’t matter.
(5)含主语从句的复合句是疑问句时,主语从句不可提前。例如:
       正确表达:Is it likely that it will rain in the evening?
       错误表达:Is that will rain in the evening likely?

4. what 与that 在引导主语从句时的区别
    what 引导主语从句时在句时在从句中充当句子成分,如主语.宾语.表语,而that 则不然。例如:
    a) What you said yesterday is right.
    b) That she is still alive is a consolation