主语从句，即在复杂句中充当主语成分的句子。
例如"That he finished writing the composition in such a short time surprised us all"，他在很短的时间内写完作文让我们都很惊讶。
此复合句主要描述的是某事让我们惊讶，某事就是该复合句的主语，因此“That he finished writing the composition in such a short time”便是该复合句的主语从句。
扩展资料
一、引导主语从句的连词主要有：
从属连词：that whether
连接代词：who whoever whom whose what whatever which whichever
连接副词：when where how why
二、主语从句不可位于句首的五种情况
1、if引导的主语从句不可居于复合句句首。
2、 It is said , (reported) …结构中的主语从句不可提前。
例如：It is said that President Jingo will visit our school next week.
3、 It happens…, It occurs… 结构中的主语从句不可提前。
例如：It occurred to him that he failed in the examination.
4、It doesn’t matter how/whether …结构中的主语从句不可提前。
例如：It doesn’t matter whether he is wrong or not..
5、含主语从句的复合句是疑问句时，主语从句不可提前。
例如：Is it likely that it will rain in the evening?
参考资料来源：百度百科—主语从句
主语从句，即在复合句中充当主语成分的句子。具体讲解如下：
引导主语从句的连词主要有：
从属连词：that whether
连接代词：who whoever whom whose what whatever which whichever
连接副词：when where how why whenever wherever however
1、常规主语从句，即句子在复合句中充当一个主语。主语从句的时态不受主句时态的影响和限制。
例句
（1）Who will be our monitor hasn't been decided yet.
现在还没有决定谁会成为我们的班长。
（2）Whatever you did is right.
你做的任何事都是正确的。
（3）What we need is time.
我们需要的是时间。
2、名词性从句的时态规则：
主句谓语动词是现在或将来时，从句谓语动词可以用任何需要的时态。
I wonder what he is doing now.我想知道他现在在干什么。
Do you know when and where he was born?你知道他在何时何地出身吗？
主句谓语动词是过去时，从句谓语动词一般需用过去时。
（1）从句与主句的谓语动作同时发生，从句用一般过去时或过去进行时。
I thought that he studied hard.我认为他学习很努力。
He told me his son was watching TV.他告诉我他的儿子在看电视。
（2）从句谓语动作在主句谓语动作之后，从句用过去将来时。
He said he would spend his holidays in Qingdao.他告诉我他将在青岛度假。
（3）从句谓语动作在主句谓语动作之前，从句用过去完成时。
They told me they had waited for me half an hour.他们告诉我他们已经等了我半小时了。
（4）从句如果有表示过去某一具体时间的状语，仍用过去时。
She told me her brother died in 1945.她告诉我她的哥哥（弟弟）死于1945年。
从句时态不受下列客观的条件影响。
（1）从句说明的是不受时间限制的事实或真理（客观真理）。
The teacher told us that the earth turns from west to east.老师告诉我们地球自西向东转。
He said Asia is the largest continent.他说亚洲是最大的大陆。
（2）当从句表示的是反复出现的时态不变。
She told me that she gets up at six o'clock every morning.她告诉我她每天早晨6点起床。
He asked me when the train usually starts.他向我询问火车通常何时出发。
（3）当从句的谓语动作仍在继续时，或者表示现在仍在的状态时，时态不变。
She told me the other day that she is only 10.她告诉我那天她只有10岁。
（4）当从句的动作状态还未发生，要用一般式表示。强调与现在的联系。
He said he will wait for me this evening.他说他今晚将等待我。
3、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
It is certain that he will win the match.他一定会赢得这场比赛的。
It is very likely that they will hold a meeting.他们很有可能会举行一场会议。
扩展资料
用it 作形式主语的结构
(1) It is +名词+that从句
It is a fact that … 事实是……
It is an honor that …非常荣幸
It is common knowledge that …是常识
(2) it is +形容词+that从句
It is natural that… 很自然……
It is strange that… 奇怪的是……
(3) it +不及物动词+that从句
It seems that… 似乎……
It happened that… 碰巧……
(4) it is+过去分词+that从句
It is reported that… 据报道……
It has been proved that… 已证实……
参考资料来源：百度百科-主语从句
第一部分：常规主语从句，即句子在复合句中充当一个主语
（1）That 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.
（5）What caused the accident remains unknown.
（6）Whatever you did is right.
（7）Whose watch was lost is unknown.
（8）What we need is time.
（9）What we need are good doctors.
小结:
（1)引导主语从句连词有that,whether,who,what,whatever等
（2)连词位于句首不能省略
（3)主语从句大多数情况下视为三单,但也有例外,如例9
主语搁置句末
（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.
（6）It is strange that the man should have stuck to his silly ideas.
（7）It is a pity that we won't be able to go to the south to spend our summer vacation.
（8）It is still a mystery what caused the accident.
（9）It is said that he has gone to shanghai.(=He is said to have gone to shanghai)
（10）It is known to all that the gun powder was first invented by the Chinese.
（11）It is suggested that the work should be done with great care.
（12）It seems that he has seen the film.(=He seems to have seen the film)
（13）It happened that the two cheats were there. (=The two cheats happened to be there)
小结:
（1)以that 引出的主语从句,常以形式主语it引导. It is +形容词/名词/某些动词ed + that 从句.
（2) 在有些that从句中要用虚拟语气 (should+do/should+have done)例句4,5,6,11.
主语从句与宾语从句两者的用法和在句中的作用是不同的，下面从它们的各自用法，介绍：）
You are right.
主语 系 表语
What you do is right.
主语 系 表语