⒈It + is/was + adj./n. (形容词或名词） +to do sth.

2.It + v. + to do sth.

3.Its +n.（名词或名词短语）/adj.（形容词）+ doing sth.

形式主语：It +be+(the)+adj.+for sb.+to do sth.

例句：

It is nice talking to you. 跟你交谈真是愉快。

It's foolish behaving like that. 这样的表现是很愚蠢的。

It is useless doing that. 那样做没用。

It's an awful job doing this. 做这事真是一件可怕的差事。

It's fun doing this. 做这事真有趣。

It is not an easy task doing this work. 做这工作真非易事。