It 用于强调结构,其强调的部分可以是主语、宾语或副词短语,但不能用这种结构强调动词,例如下面这句:
It was the Minster of Education that my mother threw an egg at yesterday.
我母亲昨天是向教育部长扔了一个臭鸡蛋。

我们再来看一个比较复杂的强调结构:

Unfortunately it is those places that are best avoided all citizens that are now capturing the popular imagination,places that offer bomb-making instructions,pornography,advice on how to steal credit cards.

解析:本句是一个强调句型,主要结构是it is those places ...that are now capturing the popular imagination,强调的是those places。

译文:不幸的是,正是这些人们应该避开的地方使得人们心驰神往。这些地方教你如何制造炸弹、为你提供淫秽作品、告诉你如何盗用信用卡。

It 充当形式主语或形式宾语时,在句中指代的对象的通常形式为:不定式、动名词或名词性从句。

在阅读中,只有弄清it 在句中的功能才能正确理解句子的含义,尤其是当it 与所指代的成分相分离,或一句话中多次出现it时,更要弄清楚他们在句子中的作用。

It is flattering to believe that their ideas are too profound to be espressed so clearly that all who run may read,and naturally it does not occur to such writes that the fault is with their own minds,which have not the faculty of precise reflection.

解析:It is flattering...read和it does not occur...reflection是并列句。It is flattering...中的it是形式主语,真正的主语是to believe...may read。其中,that their ideas...may read是believe的宾语从句。it does not occur...中的it 也是形式主语,指代的是从句that the fault...reflection.

译文:认为自己的思想太深奥,以至于不可能表达清楚使任何人都能理解,这是一种自我安慰。这样的作家当然不会想到,问题出在自己的头脑缺乏精确思考能力。