1、It除了代替人和物以外，还可以作形式主语。而真正的主语(不定式、动名词或从句)则放于谓语或表语之后。

例如：It is nor easy to finish the work in two days.

然而有少数表语之后接动名词作真正的主语。这些表语是：无助(no help)、无用(no use)、没好处(no good);工作(hard work)、费时(a waste of time)、又危险(a danger)。

例如：A、It is no use crying over spilt milk.

B、It is a waste of time waiting for him.

2、It还可以作形式宾语。通常下列动词后面可接it作形式宾语：2f2tcjm(find，feel,think,take,consider，judge, make)。

例如：A、He made it clear that he was not interested in this subject.

B、I think it no use arguing with him.

3、It用于强调句式。要强调句子的某一部分(主语、宾语、 状语)，可以把it当作先行词。这种句子的结构是：It is(was)+ 被强调部分+that(who)+句子的其余部分。

例如：A、It iS Professor Lin who teaches us English—(强调主语)

B、It was in Shanghai that l saw the film.—(强调状语)

C、It was in 1990 that I worked in the factory.(同上)

但要注意与定语从句的区别。

例如：D、It was 1990 when I worked in the factory.(定语从句)

在强调句式里，我们把强调结构It is(was)…that除去，句子还很完整。如例句C。而例句D就不能。

四、倒装结构

学生容易混淆的是全部倒装与部分倒装。如何区分之，编个顺口溜：副(adv.)介(prep.)提前全倒装，其它句式部分倒;否定提前倒助动，让步状语倒表语;复合句式倒主句，不

倒装的属特殊。下面举例说明：

A、Here comes the bus.(副词提前，全倒装)

B、Here he comes.(代词作主语，不倒装)

C、In front of the house lies a garden.(介词短语提前，全倒装)

D、Never shall I do this again.(否定词提前，部分倒装)

E、Young as he is ,he knows a lot.(让步状语从句，表语倒装)

F、Only when he told me did I realize what trouble he was in.(only 修饰状语，主句倒装)

G、Only he can save the patient.(only修饰主语.不倒装)

H、Not only will help be given to people,but also medical treatment will be provided.(否定词提前，部分倒装)

I、Not only he but also we like sports.(连接两个主语，不倒装)