1. 基本用法
当不定式、动名词、从句等复杂成分用作句子主语时，为保持句子平衡，通常把真正的主语放在句末，而在句首使用形式主语it。如：
● It’s very important to remember this. 记住这一点很重要。
● It’s hard work climbing mountains. 爬山是费劲的事。
● It’s unknown when he will come. 他什么时候来还不知道。
2. 用作形式主语的的重要句型
① It + be + adj. for (of) sb to do sth 某人做某事……
● It is hard for him to make up his mind. 他很难下定决心。
● It was foolish of her to say such a thing. 她说那样的话，真是太蠢了。
【说明】介词 of 与 for 的区别是：of 用于指某人的性格、属性、特征等，介词for表示对象，意为“对……来说”
②.It takes sb + 时间段 + to do sth. 某人做某事花了……时间
● It takes years to master a new language.
要花多年的时间才能掌握一门新的语言。
【说明】此句型可以有以下多变种变体：It took me an hour to write the letter. =The letter took me an hour (to write). =I took an hour to write the letter. 我写这封信花了一个小时。
③.It is up to sb to do sth. 该由某人做某事、做某事是某人的义务/职责/责任
It's up to you to decide how to get there.
由你来决定怎样到达那儿.
It's up to her to lock the door.
锁门是她的职责./锁门由她来负责.
④.it looks(seems, appears, happens, occurs) that [as if]… 似乎……
It seems as though he didn't recognize me.
他好像不认识我似的。
⑤.If it were not for… / If it hadn’t been for… 若不是因为……
If it were not for their help, we couldn’t have got over the difficulties.
要不是他们帮助，这些困难我们不克服不了的。