主语从句从字意上上就是可以做主语的句子。
下面的这些很系统，也就是说挺多的。看的时候耐心的话，最好一遍过。
英语主语从句
主语从句指一个句子在复合句中充当一个主语。
一. That陈述结构在主语从句中
that在名词性从句中不充当任何成分，无词义，只起连接词作用。谓语用三人称单数。
That light travels in straight lines is known to all.
That we need more equipment is quite obvious.
二. Wh疑问结构在主语从句中
主语从句要求使用陈述句语序，谓语用三人称单数。
What，who, which, when, where, why, how, whether等连接词，有疑问含义，充当各种成分。
What he said/did was right.
When we shall hold our sports meet is not decided.
Who broke the glass yesterday is not clear.
Which car you will choose to buy makes no difference.
Where she has gone is not known yet.
Why he did it will remain a puzzle for ever.
How he was successful is still a puzzle.
Whether we will hold a party in the open air tomorrow depends on the weather.不能用if.
Whether he will join us won't make too much difference.
三. What专项讲解
What he is thinking is unknown.
What he said was reasonable.
What we need is a book/are good doctors.
What he was doing/has done was right.
What we called computer was invented 50 years ago.
What was called computer was invented 50 years ago.
We finally reached what was called wasteland.宾语从句
We finally reached what was wasteland.宾语从句
What seemed like 3 years was indeed 3 days.
It is different from what it used to be.宾语从句
The village was different from what it is now.
What surprised me most was that the little girl could play the violin so well.
四. -ever关系结构在主语从句中
whatever, whoever, whichever, wherever,whenever 不含疑问意义，作主语、宾语、表语等。
whatever = anything that; whoever = anyone who; whichever=anything which;
Whenever=the time when; wherever=the place where。
What he said was right. Whatever he said was right.
You must do whatever is best for you. 宾语从句
Whoever wants it may have it.
Whichever he likes will be given him.
Wherever you are will not affect my love for you.
Whenever you are ready will be fine。
要注意和whatever, whoever引导的状语从句的区别。
Whoever breaks the law should be punished. （主语从句）
(＝Anyone who breaks the law should be punished.)
Whoever breaks the law, he should be punished. （状语从句）
(＝No matter who breaks the law, he should be punished.)
Whatever happened, he would not mind.
Whichever side wins, I shall be satisfied.
We must camp wherever we can get water.
Whenever we see him we speak to him.
五. It形式主语构成的主语从句
1. that引导的主语从句，用it作形式主语。当主语从句不太长时，“that”可以省略
It is well-known that the earth moves around the sun.
It's clear (that) he has done his best.
It's a wonder (that) he didn't fail.
2. wh疑问词引导的主语从句，用it作形式主语。
Who was responsible for the accident is not yet clear.
It is not clear who was responsible for the accident.
It is not known when he did it.
It is not important whether you are a male or a female in the job.
3. 常见用it作形式主语的复合句结构
(1). It+be+形容词+that从句。
(obvious, true, likely, certain, probable, )
It is certain that she will do well in her exam.
It is probable that he told her everything.
I think it is likely that I will arrive tomorrow.
It is obvious that you are wrong.
It is true that there are only a few things that can make me happy right now.
以下这些词后的从句用虚拟语气，为“（should）＋动词原形”：
clear , true, wonderful , possible, necessary, important, impossible, strange, natural,
essential, surprising, unthinkable, unbelievable, incredible, good ,wrong ,proper
It is necessary that you (should)master the computer.
It is important that a student learn English well.
It’s clear that they badly need help.
(2). It+be+名词词组+that从句。
(a shame ,no wonder, an honour, a good thing, a pity, no surprise, 等)
It’s a pity that we can’t go. It’s no surprise that our team should have won the game.
(3). It+be+P.P+that从句。
(said, reported, thought, expected, decided, announced, arranged, etc.)
It is said that Mr. Green has arrived in Beijing.
It is reported that China has sent another man-made earth satellite into orbit.
It is v-ed that … (should)…用虚拟语气
(verb=demand, request, require, order, suggest, advise, recommend
It is suggested that they should begin with the third question.
(4). It+seems+that从句。(seem后接that从句或as if从句)。
(look, seem, appear, happen, occur, follow, etc.)
It seems that Alice is not coming to the party at all.
It happened that I was out that day.
(5). 几种特殊的形式主语 或固定句型
It doesn’t matter whether she will come or not.
It makes no difference whether he will attend the meeting or not.
if it were not for…(若不是因为)， = Were it not for…
it’s time that…(该…了)
6. 主语从句和定语从句区别
it作形式主语代替主语从句时，要注意和as引导的定语从句的区别。试比较：
It was reported that the US was under the terrorist attack.（主语从句，有that，无逗号）
As was reported, the US was under the terrorist attack. （定语从句，无that，有逗号）
7. 主语从句与强调句的区别
(1). 在强调句中，可以去掉It was ...that还像个句子。主语从句中不行。
(2). 主语从句引导词：what, who, whom, whose, which, whatever, whoever, whichever
强调句的引导词只有：that, who。
主语从句：
It was strange that he had made a mistake.
It is immaterial where or when he goes. 无关紧要的
It is said/reported that he’s got married.
强调句：
It was she that had been wrong. （强调主语）
It was this novel that they talked about last night. （强调宾语）
It was in London that I first saw her. （强调地点状语）
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