一、主語從句是在復合句中充當主語的從句，通常放在主句謂語動詞之前或由形式主語it代替，而本身放在句子末尾。
1. It 作形式主語和it引導強調句的比較：
It 作形式主語代替主語從句，主要是為了平衡句子結構，主語從句的連接詞沒有變化。而it引導的強調句則是對句子某一部分進行強調，無論強調的是什么成分，都可用連詞that。被強調部分指人是也可用who/whom。例如：
It is a pity that you didn’t go to see the film.
你沒有去看這個電影，多可惜啊！
It doesn’t interest me whether you succeed or not.
對于你是否成功，我并不感興趣。
It is in the morning that the murder took place.
兇殺案發(fā)生的時間是在早上。
It is John that broke the window.
打碎窗戶的是約翰。
2.用it 作形式主語的結構
(1)It is ＋名詞＋從句
It is a fact that … 事實是… It is an honor that …非常榮幸 It is common knowledge that …是常識
It’s a pity that you missed the exciting football match.
多么可惜啊，你錯過了一場精彩的足球比賽。
(2)it is ＋形容詞＋從句
It is natural that… 很自然… It is strange that… 奇怪的是…
It is uncertain whether he can come to Jenny’s birthday party or not.
他來不來參加簡的生日派對，這個不確定。
(3)it is ＋不及物動詞＋從句
It seems that… 似乎… It happened that… 碰巧…
It seemed that he didn’t tell the truth.
似乎他沒有講真話。
It happened that I was out when he called.
碰巧，他打電話的時候我在外面。
(4)it ＋過去分詞＋從句
It is reported that… 據報道… It has been proved that… 已證實…
It is reported that 16 people were killed in the earthquake.
據報道，在這場地震中，有16人遇難。
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