基本信息
grammar
主语从句
在复合句中充当句子主语的从句叫主语从句。
【要点精讲】
一、whether, who, what, which, when, where, how, why等都可引导主语从句。如：
whether we will go for an outing tomorrow remains unknown.
what i need most now is more time.
二、that引导主语从句时，that在句中不作句子成分，且无实际意义。当that引导的主语从句置于句首时，that不能省略；如果用it作形式主语，而把主语从句放在句末时，that可以省略。如：
that i will take an english test next week is certain.
it is a pity (that) you made such a careless mistake.
三、it作形式主语的几种常见句型
①it + be + n. +主语从句。常用于此句型中的名词有fact, rule, pleasure, truth, pity等。如：
it is a fact that english is being accepted as an international language.
②it + be + adj. +主语从句。常用于此句型中的形容词有true, important, necessary, clear, certain等。如：
it’s true that the earth is round.
③it + v. +主语从句。常用于此句型中的动词有seem, appear, happen, follow等。如：
it appeared that she was older than she really was.
④it + be +动词-ed形式+主语从句。常用于此句型中的动词-ed形式有known, said, proved, asked, suggested等。如：
it’s well known that thomas edison invented the electric lamp.
注意：if和whether都可引导主语从句，但if不能引导位于句首的主语从句。但如果用it作形式主语，而把主语从句放在句末时，也可以用if引导。如：
whether tom really saw sharon was doubtful.
it was doubtful if / whether tom really saw sharon.
表语从句
