从句作主语就是主语从句
常规主语从句,即句子在复合句中充当一个主语
1That he finished writing the composition in such a short time surprised us all.
（2）Whether we will go for an outing tomorrow remains unknown.
（3）Who will be our monitor hasn't been decided yet.
（4） Whom we must study for is a question of great importance.
为了防止句子头重脚轻,通常把形式主语it放在主语位置,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. （5）It is important that we all should attend the meeting