I think that it is important to master a foreign language.(改为同义句)

题目

I think that it is important to master a foreign language.(改为同义句)
I think _____ _____ ______ _______ a foreign language.
important to master,可是还是不明白为什么,这涉及到什么形式主语、形式宾语之类的,我不太明白,

答案

原句I think that it is important to master a foreign language.的it是形式主语.
I think it important to master a foreign language.it是形式宾语
1.it做形式主语
It is +adj.+(for sb.) +to do sth.
It is time to do/for/that.
2.it做形式宾语