主语从句是一个重要的语法点，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,本文对主语从句语法做了一个详细的梳理，帮助大家梳理主语从句的语序，连接词的选用等内容，对于it构成的主语从句做了详细的解释。,以便帮助大家掌握这一重要知识点。
一.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。
二. 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. ）
三. 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。
另外，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…）th
It is necessary (important, natural, strange, etc.) that …
It is suggested (requested, proposed, desired, etc.) 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。
【主语从句】
主语从句是指在句子中做主语的从句。引导主语从句的词有连词that, whether；连接代词who, what, which以及其强调形式whoever, whatever, whichever；连接副词when, where, how, why等。
例如：That she was able to come made us very happy.
Whether she will come or not is still a question.
What we need is more time.
Where she has gone is a mystery.
在主语从句中，为了保持句子的平衡，我们常常把主语从句后置，而以形式主语it代替。
例如：That he misunderstood me is obvious. 通常说成：
It is obvious that he misunderstood me.
When the plane is to take off has not been announced. 通常说成：
It has not been announced when the plane is to take off.
引导主语从句位于句首的连词不能用if，可以使用whether。
例如：If he comes or not doesn’t matter. 应该为：
It doesn’t matter if he comes or not.
或 Whether he comes or not doesn’t matter.
that引导的主语从句放在句首时，that不能省略。
例如：That he was chosen made us very happy.
注意这种结构构成的一些常见句型，如：
It is said / reported that …. 据说/据报道……。
It must be pointed out that …. 必须指出的是……。
It is likely that …. 有可能……。
It happens that …. 碰巧……。
what与that在引导主语从句时的区别：what在从句中充当句子成分，如主语、宾语、表语，而that在从句中不做成分。
例如：What you said yesterday is right.
（what在从句中作said的宾语）
That she is still alive is surprising.
（that在从句中不做成分，从句she is still alive不缺少成分）
用来作主语的句子叫做主语从句。如：
(1)When he was born is unknown. 他生于何时还不知道。
(2)What he did last night is being investigated. 他昨天晚上干了些什么正在调查之中。
在主语从句中须注意：
1. 主语从句一般用it作形式主语，放在句首，而将主语从句放到句末。如：
(1)It is possible that he has stolen the car. 很可能他偷了小车。
(2)Isn’t it strange that he should not have passed the test? 他测试没有通过，难道不奇怪吗？
2. if引导主语从句时，只能用it作形式主语，放在句首，而将if引导的主语从句放到句末。如：
It is uncertain if he will leave for Beijing tomorrow. 明天他是否去北京还不敢肯定。
3. that引导主语从句放在句首时，that不可省，但在句末口语中可省。如：
That he was ill yesterday is known now. 大家都已知道他昨天生了病。
4. 连接代词引导主语从句只能放在句首。如；
(1)Who has broken the glass is unknown. 谁打破了玻璃现在还不知道。
(2)What he wants is all here. 他所要的东西都在这里。
5. 主语从句中主句的谓语动词一般用第三人称单数形式，但what引导主语从句的主句谓语动词的形式须根据句子的意思来判断。如()：
(1)How he worked it out is still a secret. 他是如何把它做出来的还是一个秘密
(2)What they makes in this factory are TV sets. 他们在工厂里所做的东西就是电视机。 |
