英语中it既可以充当形式主语也可以充当形式宾语，将真正的主语或者真正的宾语to do不定式放在后面。例如
It is becoming more and more important for us to study English.形式主语。
It is necessary for us to go there by air at once.形式主语。
The students find it convenient to go there by boat.形式宾语。
Our school has made it a rule for us to get up early.形式宾语
it's作形式宾语的情况
It’s做形式宾语时，有两个比较有用的句型：
1、It’s ＋形容词＋for sb. ＋to do sth.对于某人来说，做某事是很…
2、It’s＋名词＋to do sth.
做某事是…
