这是主语从句：为了防止句子头重脚轻,通常把形式主语it放在句首位置,真正主语搁置于句末
（1）It is certain that he will win the match.
（2）It is true that he has made a very important discovery in
chemistry.
（3）It is very likely that they will hold a meeting.
（4）It is strange that he should do that.