一.定义：在复合句中起主语作用从句

二.结构：关联词➕主语从句剩余部分

三.关联词：

1.从属连词：that, whether

2.连接代词：who, what, which, wh-ever

3.连接副词：when, where, how, why, wh-ever

※注意：主语从句放在句首时，整个句子会显得笨重，为了使句子结构平衡，优美，常把主语从句移到后边，前边用引导词"it"做形式主语。

四.it 作形式主语的运用

1.It➕is/was➕名词（名词词组）➕主语从句

It's a fact …

It's a pity …

It's a shame …

It's an honour …

It's a question …  等等

E.g.

It’s a pity that you missed the exciting football match.

It remains a question whether he will come or not.

2.It➕is/was➕形容词➕主语从句

It's natural …

It's  obvious…

It's fortunate …

It's possible…

It's unlikely…

3.It➕is/was➕不及物动词（vt.）➕主语从句

It's seems…

It's happened…

It's appears…

It's possible…

It's turns out…

4.It➕is/was➕过去分词➕主语从句

It's known…

It's said…

It's reported…

It's decided…

It's announced…

5.其他形式

It doesn't matter…是无关紧要的

It makes no difderence…毫无区别

It is of little consquence that…无关紧要

It suddenly struck me（occurred me）that…

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