一.it 作形式主语
1.it is +形容词/名词 （for/of sb ）to do sth
eg.It is important for us to protect the environment.
It is kind of you to help me.
2.It is ＋形容词／名词十that从
eg：It is necessary that we should keep tha balance of nature.
3.It is +过去分词+that 从句
eg：It's said that he has gone there.
二.作形式宾语
find/think/make…+ it+to do sth
eg:I found it easy to work with him.
