主语从句：在句子中起主语作用的从句。
The book is popular.（the book是主语；is是系动词；popular是表语）
The book that Mr.black wrote it is popular.
(is是系动词，popular是表语；主语是受欢迎的，那么这个句子中主语就是Mr.black写的这本书。)
The earth is round.That the earth is round is known to all.(be known to 为…所熟知)
(that the earth is round 为起主语作用的从句)
that 引导主语从句，在从句中不作任何成分，无词汇意义。
That 引导主语从句位于句首：That she cheats on you is for sure.(cheat on 对某人不忠实，作弊)
句子中用形式主语it代替，that从句位于句末。
It+系动词+形容词/名词/动词+that 从句：it is obvious that we have made it.(make it 成功)
it is my hope that the whole world becomes peace.
It is reported that no passengers were injured in the accident.
It turns out that our team has won the game.
【小tip】
在It作形式主语时，下列示例从句that中有should，则should可以省略。it is+necessary/important/surprising/strange/unthinkable/unbelievable/incredible+that从句（should do）should可以省略。
It is suggested/advised/ordered/requested/required/insisted/demanded/that从句（should do）should可以省略
It is suggested that we should spend more time in practicing English.
It is suggested that we spend more time in practicing English.
It is important that we should have to face reality.
It is important that we have to face reality.
Whether引导主语从句位于句首。
Whether he accepts it or not doesn’t affect me.
【小tip】
若翻译为是否，whether和if均可以表达。但是if不可以引导位于句首的主语从句，如果用It作形式主语，置于后面的从句可以用If引导。
It is uncertain that whether/if he will pass the exam.
连接词引导主语从句：连接代词和连接副词均可。
连接代词：who/whom/whose/what/whoever/whomever/whosever/whatever等。
连接副词：when/where/how/why/whenever/wherever/however等。
who did it is still a mystery.
When we should go back is a problem.
Why he got a divorce is not clear
