一．主語(yǔ)從句
主語(yǔ)從句是在復(fù)合句中充當(dāng)主語(yǔ)的從句，通常放在主句謂語(yǔ)動(dòng)詞之前或由形式主語(yǔ)it代替，而本身放在句子末尾。
1. It 作形式主語(yǔ)和it引導(dǎo)強(qiáng)調(diào)句的比較
It 作形式主語(yǔ)代替主語(yǔ)從句，主要是為了平衡句子結(jié)構(gòu)，主語(yǔ)從句的連接詞沒(méi)有變化。而it引導(dǎo)的強(qiáng)調(diào)句則是對(duì)句子某一部分進(jìn)行強(qiáng)調(diào)，無(wú)論強(qiáng)調(diào)的是什么成分，都可用連詞that。被強(qiáng)調(diào)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 作形式主語(yǔ)的結(jié)構(gòu)
(1) It is ＋名詞＋從句
It is a fact that … 事實(shí)是…
It is an honor that …非常榮幸
It is common knowledge that …是常識(shí)
(2) it is ＋形容詞＋從句
It is natural that… 很自然…
It is strange that… 奇怪的是…
(3) it is ＋不及物動(dòng)詞＋從句
It seems that… 似乎…
It happened that… 碰巧…
