表语从句的概念：

用作表语的从句叫作表语从句，它位于主句中的连系动词之后。引导表语从句的词有
从属连词that、whether、as though(if)；
关系代词who, what, which, whom, whose, whatever, whoever, whomever, whichever等；
关系副词when, where, why, how, however, when。

表语从句就是用一个句子作为表语。说明主语是什么或者怎么样，由名词、形容词或相当于名词
或形容词的词或短语充当，和连系动词一起构成谓语。
1）that引导的宾语从句(that经常可以被省略)， 例如：
His mother said (that) Saul Grant was a music critic. 他母亲说索尔?格兰特是个音乐评论家。
把握that引导的宾语从句要注意以下特殊情况：
（1）宾语从句(通常是由that引导的宾语从句)常置于句末并用it作形式宾语。例如：
     We think it important that Mary should tell the truth. 我们认为玛丽说出实情很重要。
     I have made it clear that we’ll sign the contract with your company.
     我已讲得很清楚： 我方将与贵公司签订这个合约。
（2）that 从句作介词宾语只用于except that和in that结构。例如： 
     His report is correct except that some details are omitted. 
     除了有些细节未提到之外， 他的报道是正确的。
     Criticism and self-criticism is necessary in that it helps us to correct our mistakes.
     批评和自我批评是必要的， 因为它能帮助我们改正错误。
（3）allow, refuse, let, like, cause, force, admire, celebrate, dislike, love, help, take, 
     forgive等及物动词后可以接不定式或动名词作宾语， 但不可以接that引导的宾语从句。例如：
     I admire their winning the match. (√)
     I admire that they won the match. (?菖)
2）与疑问词形式相同的连词引导的宾语从句， 这包括：
（1）动词(＋间接宾语)＋与疑问词形式相同的连词引导的宾语从句。常用于这种句型的动词有see, know,
     wonder, show, ask, tell, decide, find out, imagine等。例如：
     Tell me how you went to school. 告诉我你怎么去上学的。
     John wondered why he was wanted by the police, but he went to the station yesterday.
     约翰不知道为什么警察局要他去， 但昨天他还是去了。
（2）介词＋与疑问词形式相同的连词引导的宾语从句。例如： 
     I am not interested in what he is doing. 我对他做的事不感兴趣。
     He will write a book on how Chinese people learn English. 他将写一本中国人如何学英语的书。
（3）be＋形容词＋与疑问词形式相同的连词引导的宾语从句(这种结构中的“be＋形容词”可以被视为
     发挥着及物动词的作用， 其后的从句具有宾语从句的性质)， 例如：
     Are you certain that you'll get there in time? 你有把握及时赶到那里吗？
     He was disappointed that he failed to get the master degree. 他很失望没有获得硕士学位。
3） whether与if引导宾语从句时一般可以互换， 例如：
    The boss asked me whether/if she was qualified for the task. 老板问我她是否胜任这项任务。

表语从句用法：

1、表语从句的引导词：
引导表语从句的词有连词that, whether，连接代词和连接副词，关系代词型what，以及as if,as though,because等连词：
如：The fact is that he doesn't really try. 事实是他没有做真正的努力。
        The question is whether the film is worth seeing. 问题是这部电影是否值得看。
        The problem is how we can find him. 问题是我们如何找到他。
        That was when I was fifteen. 这是我15岁时发生的事。 
        It isn't as if you were going away for ever. 又不是你离开不回来了。
        It is because I love you too much. 那是因为我太爱你了。
注：whether可此导表语从句，但与之同义的if 却通常不用于引导表语从句；because可引导表语从句，但与之同义的since, as, for等也不用于引导表语从句。

2、连词that的省略问题引导表语从句的that通常不省略，但在口语或非正式文体中有时也可省略：
如：My idea is(that) we should do it right away. 我的意见是我们应该马上干。
        The trouble is(that) he is ill. 糟糕的

表语从句用法拓展：

名词性从句在be等系动词后作表语时被称为表语从句，表语从句只能置于主句之后，而主句的动词只能是联系动词。
例如：The problem is how we can get the things we need. 问题是我们怎样能弄到我们需要的东西。(how在表语从句中充当方式状语) 　　
            The scissors are not what I need. 这把剪刀不是我所需要的。(what在表语从句中充当宾语) 　　
            What I told him was that I would find him a good play.
            我告诉他的是我会给他找个好剧本。(what在主语从句中作直接宾语，that作为表语从句的引导词在该表语从句中不充当句子成分，不能省略) 　　
            That is what I want to tell you. 那就是我想要对你讲的。(what在表语从句中充当直接宾语) 　　
            That is why she failed to pass the exam. 那就是她考试不及格的原因。(why在表语从句中充当原因状语)
注意：“That is why...”是常用句型，意为“这就是……的原因/因此……”，其中why引导的名词性从句在句中作表语，该句型通常用于针对前面已经说明过的原因进行总结：
如：That is why you see this old woman before you know, Jeanne.珍妮，这就是现在这个老太婆出现在你面前的原因。(前文提到Jeanne对老妇人显得苍老憔悴深感诧异，说话人对她讲述了其中的原因之后，用这一句来进行概括)。//ThatiswhyIcame.这就是我来的原因。下面是两个与“That is why...”形式相似的结构，它们与“That is why...”结构之间的关系要能够辨析清楚：　　
（1）“That is why...”与“That is the reason why...”同义，只不过从语法结构上讲，“That is the reason why...”中why引导的是—个定语从句，将其中的the reason去掉则与“That is why...”结构一样：
如：That is(the reason) why I can not agree. 这就是我不能同意的理由。　　
（2）“That is because...”句型中从属连词because引导的名词性从句在此作表语，这也是个常用句型，意为“这就是为什么……/因为……”。“That is because...”与“That is why...”之间的不同在于“That is because...”指原因或理由，“That is why...”则指由于各种原因所造成的后果：
如：He did not see the film last night.
        That is because he had to help his little sister with her homework. 昨天晚上他没有去看电影，那是因为他得帮助他的妹妹做作业。(第一句话说明结果，第二句话说明原因) 　　          
        He had seen the film before. That is why he did not see it last night. 他以前曾看过那部电影，因此他昨天晚上没有去看。(第一句话说明原因，第二句话说明结果)

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