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.
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 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…
(4) it ＋过去分词＋从句
   It is reported that… 据报道…
   It has been proved that… 已证实…
3. 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：
1) What you said yesterday is right.
2) That she is still alive is a consolation.
二．宾语从句
   宾语从句就是在复合句中作宾语的名词性从句，通常放在主句谓语动词（及物动词）或介词之后。
1. 作动词的宾语
(1) 由that引导的宾语从句（that 通常可以省略）,例如：
   I heard that be joined the army.
(2) 由what, whether (if) 引导的宾语从句，例如：
   1) She did not know what had happened.
   2) I wonder whether you can change this note for me.
(3) 动词＋间接宾语＋宾语从句。例如：
   She told me that she would accept my invitation.
2. 作介词的宾语
  例如：Our success depends upon how well we can cooperate with one another.
3. 作形容词的宾语
例如：I am afraid (that) I’ve made a mistake.
  that 引导的从句常跟在下列形容词后作宾语：
  anxious, aware, certain, confident, convinced, determined, glad, proud, surprised, worried, sorry, thankful, ashamed, disappointed, annoyed, pleased, hurt, satisfied, content 等。也可以将此类词后的that 从句的看作原因状语从句。
4. it可以作为形式宾语
   it 不仅可以作为形式主语，还可以作为形式宾语而真正的宾语that 从句则放在句尾，特别是在带复合宾语的句子中。 例如：
   We heard it that she would get married next month..
5. 后边不能直接跟that 从句的动词
   这类动词有allow, refuse, let, like, cause, force, admire, condemn, celebrate, dislike, love, help, take, forgive等。这类词后可以用不定式或动名词作宾语，但不可以用that引导的宾语从句。例如：
I admire their winning the match.
6. 不可用that从句作直接宾语的动词
   有些动词不可用于“动词＋间接宾语＋that从句“结构中，常见的有order, accuse, refuse, impress, forgive, blame, denounce, advise, congratulate等。