1. 主语从句的语序
主语从句要求使用陈述句语序，而非一般疑问句语序。如：
What surprised me most was that the little girl could play the violin so well.
使我感到惊讶的是这个小女孩的小提琴拉得太好了。
How he was successful is still a puzzle.
他是如何成功的仍然是个谜。

2. 连接词的选用
（1）that和what的选用
that和what都可引导主语从句。what除起连接作用外，还在名词性从句中充当某些成分，可作从句的主语、宾语或表语。而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。如：
What he wants is a book. 他想要的是一本书。
That light travels in straight lines is known to all. 众所周知，光线沿直线运行。
（2）if和whether的选用
引导主语从句，不能用if，只能用whether。如：
Whether we will hold a party in the open air tomorrow depends on the weather.
我们明天是否在户外开晚会要看天气而定。
（3）其它连接代词和副词的选用
根据主语从句的具体意义，正确的选择who, which, when, where, why, how等连接词，这些连接词既有疑问含义，又起连接作用，同时在从句中充当各种成分。如：
When we shall hold our sports meet is not decided. 我们何时举行运动会还没有决定。
Who broke the glass yesterday is not clear. 还不清楚昨天谁打破了玻璃。
Which car you will choose to buy makes no difference. 你决定买哪一辆车都不会有任何区别。
（4）whatever / whoever的功用
whatever, whoever在主语从句中不含疑问意义。它引导主语从句，并在从句中作主语、宾语、表语等。whatever = anything that; whoever = anyone who。要注意和whatever, whoever引导的让步状语从句的区别。如：
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. （让步状语从句）（＝No matter who breaks the law, he should be punished. ）
3. it构成的主语从句
（1）由连词that引导的主语从句，在大多数情况下会放到句子的后面，而用代词it作形式主语。如：
It is well-known that the earth moves around the sun. 众所周知，地球围绕太阳转。
需要注意的是，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）上两句意为“据报道，美国遭到恐怖分子的袭击”。
（2）常见用it作形式主语的复合句结构
<a>It is a fact （a good idea / a pity / a shame / no wonder / good news…）that…如：
It's a pity that you missed the film. 你没有看那部电影真是太遗憾了。
<b> It is necessary （clear / true / strange/ important /wonderful / possible / likely…）that…
需要注意的是，这类主语从句中，谓语动词很多为“（should）＋动词原形”，即要用虚拟语气。如：
It is necessary that you （should）master the computer. 你很有必要掌握电脑。
It is important that a student learn English well. 学生学好英语很重要。
It's clear that they badly need help. 很明显，他们急需援助。
It is likely that a hurricane will arrive soon. 飓风很可能马上就要到达了。
<c>It is reported （well-known / hoped / thought / expected /said /believed/decided /suggested /ordered…）that…如：
It is said that he was killed in the earthquake. 据说他在地震中丧生了。
<d>It seems （happened / appears / doesn't matter / makes no difference / …）that …如：
It seems that they will win the game. 看起来他们好像会赢得这场比赛。
It makes no difference whether he will attend the meeting or not. 他是否会参加会议都无关紧要。

4. 必背
用形式主语it引导的主语从句相当多，有些已形成固定的语法和译法。常见的有：
It is said that... 据说……  It is reported that...  据报导……
It is well known that...  众所周知……  It is announced that... 据宣布……
It is believed that...  人们相信……  It is thought that...  人们认为……
It is understood that...  自不待言……  It must be pointed out that...  必须指出……
It must be admitted that...  必须承认……

5. 否定转移
1) 将think, believe, suppose, expect, fancy, imagine等动词后面宾语从句的否定词转移到主句中，即主句的谓语动词用否定式，而从句的谓语动词用肯定式。
I don't think I know you.　我想我并不认识你。
I don' t believe he will come.　我相信他不回来。
注意：若谓语动词为hope,宾语从句中的否定词不能转移。
I hope you weren't ill. 我想你没有生病吧。
2) 将seem, appear 等后的从句的否定转移到前面。
It doesn't seem that they know where to go.看来他们不知道往哪去。
It doesn't appear that we'll have a sunny day tomorrow. 看来我们明天不会碰上好天气。
3) 有时将动名词，介词短语或整个从句的否定转变为对谓语动词的否定。
I don't remember having ever seen such a man.我记得从未见过这样一个人。 (not否定动名词短语 having…)
It's not a place where anyone would expect to see strange characters on the street. 在这里，人们不会想到在街上会碰上陌生的人。(anyone 作主语，从句中的谓语动词不能用否定形式。)
4) 有时状语或状语从句中否定可以转移到谓语动词前。
The ant is not gathering this for itself alone.　(否定状语)　蚂蚁不只是为自己采食。
He was not ready to believe something just because Aristotle said so. (否定because状语) 他并不因亚里斯多德说过如何如何，就轻信此事。
She had not been married many weeks when that man's younger brother saw her and was struck by her beauty.　(否定状语many weeks)　她结婚还不到几个月，这个人的弟弟就看见她了，并对她的美貌着了迷。

6. 典型例题
主语从句是中学阶段的重点语法项目，也是高考的热点之一。为使同学们更好地掌握其用法，本文结合高考试题，对主语从句的考查热点进行梳理，以便帮助大家明确其命题特点，掌握答题技巧。
一、恰当选用连词
1. 考查连词that, whether, what等。
that引导主语从句时,只起连接句子的作用,本身没有词义,在从句中也不作句子成分,但通常不能省略。例如:
That the college will take in more new students this year is true. 今年这所大学将招收更多新生是真的。
whether引导主语从句时,表示主语从句意义的不确定性,在从句中不作句子成分,但有词义,作"是否"解。例如:
Whether he can finish his task on time is of great importance. 他是否能按时完成任务非常重要。一般说来,已经确定的事情常由that引导,没有决定的事情常由whether引导。
what引导主语从句时,意为"......的事物",相当于the thing(s) that。
[原题再现]
① ________ you don't like him is none of my business.
A. What B. Who C. That D. Whether
② After Yang Liwei succeeded in circling the earth, ________ our astronauts desire to do is walk in space.
A. where B. what C. that D. how
答案: ① C ② B
2.考查whatever, whoever, whichever, whomever等。
这些词在从句中不含疑问意义,它们引导主语从句时,whoever=the person who或anyone who;whatever=anything that;whichever意为"无论哪一个"。
[原题再现]
________ leaves the room last ought to turn off the lights.
A. Anyone B. The person C. Whoever D. Who
答案: C
3. 考查where, when, how, why, wherever以及how many / how much / how far / how long / how soon等。
这些词本身有词义,并在从句中作状语。例如:
How they will solve the serious problem has not been decided. 他们将怎样解决这个严重问题还没有决定。
Where we can look up his address is still a problem. 我们在哪儿可以查到他的地址还是个难题。
Why he did that wasn't quite clear. 他为什么做那件事还不十分清楚。
How much water is flowing can be easily measured. 有多少水在流动很容易就能测出来。
二、it用来作形式主语
在某些情况下,可以由it作形式主语,而把真正的主语从句移到句子后面。
1. 谓语是seem, appear, be certain, be a pity, be a wonder, be one's hope, be likely等词或短语时。
[原题再现]
The Foreign Minister said, "________ our hope that the two sides will work towards peace."
A. This is B. There is C. That is D. It is
答案: D
2. 用于It is suggested / required / ordered / demanded that...句型中,动词为表示命令、建议、要求等意义的词。主语从句应使用虚拟语气,谓语动词用"should + 动词原形",should可以省略。例如:
It is suggested that you (should) spend more time in studying English. 你应该花更多的时间学习英语。
3. 用于It is important / natural / necessary / impossible that...句型中,主语从句应使用虚拟语气,谓语动词用"should + 动词原形"的形式,should有时有感情色彩。例如:
[原题再现]
It is necessary that a college student ________ at least a foreign language.
A. masters B. should master C. mastered D. will master
答案: B