名词性从句是由if, whether, that 和各种疑问词充当连接词所引导的从句，其功同名词一样。

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。被强调部分指人是也可用who/whom。例如：

    It is a pity that you didn’t go to see the film.

    It doesn’t interest me whether you succeed or not.

    It is in the morning that the murder took place.

    It is John that broke the window.

    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…