it做形式主语，避免头重脚轻

一.不定式（短语）做主语时，往往由it代替它作形式主语（但不可用that作形式主语）

It's unlike him to be late.他可不是迟到的那种人。

It will take the works over a year to build the fly-over 建造这座立交桥将花费工人们一年多的时间

It would be wrong for people to marry for money.人们为钱而结婚是错误的。（It+be+形容词+for短语+不定式）

It's a shame to say like that.那样说话是可耻的。（It+be+名词+不定式）

It is against may principles to do such a thing.做这样的事是违反我的原则的。（It+be+介词短语+不定式）

二.用于主语从句中，为了防止句子头重脚轻，通常把形式主语it放在句首位置，真正主语搁置于句末

（1）It is certain that he will win the match.

他一定会赢得这场比赛的。

（2）It is true that he has made a very important discovery in chemistry.

他的确在化学领域有了一个伟大的发现。

（3）It is very likely that they will hold a meeting.

他们很有可能会举行一场会议。

（4）It is strange that he should do that.

他竟然那样做是很奇怪的。

（5）It is important that we all should attend the meeting.

重要的是我们都应该参加会议。

（6）It is strange that the man should have stuck to his silly ideas.

真奇怪，这个人竟然坚持自己愚蠢的想法。