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1、形式主语是一个代词，用来“代”句子的真正主语，也就是说“可以实现和真正主语换位的那个IT，就是形式主语”，可是楼主所举的句子能换位吗？ 从表面上看，第一句话说成“Who did the experiment in the lab yesterday evening was my father.”好像没有什么语法错误，但是这里却存在致命的逻辑错误。 如果句子主语是Who引导的主语从句，那么动作的发出者就成了“我爸爸（who是连接代词）昨晚在实验室里做实验”。 注意：“我爸爸昨晚在实验室做实验”这是一件事，可不是一个人，所以后面怎么能接“was my father ”呢？如果这样写，那么句子直译过
2、来就是“我爸爸昨晚在实验室里做实验是我爸爸。”这语言是不是很后现代？ 那么这句话能不能改对呢？ 答案是能，而且可以改得非常漂亮，非常地道，只要加一个代词就可以了，我把它改成“He who did the experiment in the lab yesterday evening was my father.” （顺便说一句，虽然很多人，也包括绝大多数老师，都喜欢写“The man who”，但是有英文阅读习惯的人一般都认为“He/She who”的表达更加地道，当然前者也没有任何错误。） 发现了么？这么一改，“Who”引导的从句就成了定语从句，而句子的主干则成了“He was my fat
3、her.”，逻辑错误消失了。 问题2： 这里的问题和上面完全一致，所以楼主能把这两个问题捆绑起来提问也很高明。 what做连接词的时候永远等于“the thing(s) that”，那么如果用what的话，显然就是把句子当成形式主语IT引导的句子，而不是强调句了，于是它可以把主语还原，变成“The thing that attracted the audiences interest was how the young man had learned five foreign languages.”，而这句话显然是正确的。 于是，问题变成了为什么一定要把这句话看成强调句，而不能把它当成IT作形
4、式主语呢？ 答案是，因为如果不掉过来，这句话是错的。 我们退回一步，用最简单的方式还原，这句话成了“It was how the young man had learned five foreign languages the thing that attracted the audiences interest.” 楼主，这话通顺吗？是不是很别扭？能告诉我别扭在哪里吗？ 这话别扭在，IT作形式主语只能代名词性结构（从名词性短语、动名词、名词性从句），它代不了名词+定语从句的结构。 怎么改？ 在问题1中，我们添了个主语，把主语从句变成了定语从句，这里我们反其道而行之，给它去掉主语“the th
5、ing”，把定语从句还原回主语从句，于是，强调句的结构就出现了。 所以归根结底，强调句和IT作形式的主语的区别是什么？ 是that/who所引导的句子是能够独立作主，还必须修饰别人的区别；如果能够独立从当主语而不会出现逻辑错误，那它就是IT作形式主语；反之，必须通过添主语的方式把它改造成定语从句，让它修饰新添加的主语句子逻辑上才能通顺，那它就是强调句。第一部分常规主语从句，即句子在复合句中充当一个主语主语从句的时态：不受主句的时态影响和限制。（1）That he finished writing the composition in such a short time surprised us
6、 all.（2）Whether we will go for an outing tomorrow remains unknown.（3）Who will be our monitor hasnt 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）Who the watch belongs to was lost is unknown.（8）
7、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第三人称单数，但也有例外，如例（9）3第二部分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（1）It is certain that he will win the match.（2）It is true that he has made a very important discovery in chemistry
8、.（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 wont be able to go to the south to spend our summer vacation
9、.（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 happened that the t
10、wo cheats were there. (=The two cheats happened to be there)小结:（1）为避免头重脚轻，常以形式主语it引导从句。 It+形式主语谓语+从句。（2）由于主语从句一般被认为是单数形式，形式主语的谓语动词、be动词+表语 均为单数第三人称形式或过去式.主语从句的用法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来介绍。一主语从句主语从句 定义：在句子中担当主语的是一个从句，这个从句就叫做主语从句。主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。1. It 作形
11、式主语和it引导强调句的比较主语从句常用it作形式主语，一般常用句型为：It is+名词/形容词/过去分词+主语从句例如：It is still a question whether she will come or not.It is strange that you should like him.It is still unknown which team will win the match.另外，还有一些比较多见的结构：It turned out that;It has been proved that;It happened/occurred that;It is well-kno
12、wn that等等而强调句则不同，它的结构是：It+be+被强调部分+that+从句强调主语：It is the times that produce their heroes.时势造英雄强调宾语：It is English that Prof.Lin teaches us.强调状语：It was in shanghai that I saw the film.判断是否是强调句有一个方法，就是将that以后的“句子的其余部分”拿出来单独看，看有没有缺成分，缺的是什么成分，再把谓语动词后面that前面的那部分，带到你认为缺成分的地方，如果放进去是一句完整的句子了，那就说明是强调句。2. 用it
13、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3.主语
14、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 doesnt matter how/whether 结构中的主语从句不可提前。例如：It doesnt m
15、atter 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?4. What 与that 在引导主语从句时的区别What 引导主语从句时在从句中充当句子成分，如主语宾语表语，而that 则不然。例如：What you said yesterday is right.二：宾语从句用以区分主语从句的几个特征：1、引导词：whatwhichwhosewhenwhetherifwhere2语序：宾语从句必须是用陈述语句。如：I think that you must work harder.宾语从句的引导词、连接词的区别、否定转移等现象。(1)表达时间的几个句型：一般用when或者是what time：What time
