表語從句的概念：
用作表語的從句叫作表語從句，它位于主句中的連系動(dòng)詞之后。引導(dǎo)表語從句的詞有
從屬連詞that、whether、as though(if)；
關(guān)系代詞who, what, which, whom, whose, whatever, whoever, whomever, whichever等；
關(guān)系副詞when, where, why, how, however, when。
表語從句用法：
1、表語從句的引導(dǎo)詞：
引導(dǎo)表語從句的詞有連詞that, whether，連接代詞和連接副詞，關(guān)系代詞型what，以及as if,as though,because等連詞：
如：The fact is that he doesn't really try. 事實(shí)是他沒有做真正的努力。
The question is whether the film is worth seeing. 問題是這部電影是否值得看。
The problem is how we can find him. 問題是我們?nèi)绾握业剿?/p>
That was when I was fifteen. 這是我15歲時(shí)發(fā)生的事。
It isn't as if you were going away for ever. 又不是你離開不回來了。
It is because I love you too much. 那是因?yàn)槲姨珢勰懔恕?/p>
注：whether可此導(dǎo)表語從句，但與之同義的if 卻通常不用于引導(dǎo)表語從句；because可引導(dǎo)表語從句，但與之同義的since, as, for等也不用于引導(dǎo)表語從句。
2、連詞that的省略問題引導(dǎo)表語從句的that通常不省略，但在口語或非正式文體中有時(shí)也可省略：
如：My idea is(that) we should do it right away. 我的意見是我們應(yīng)該馬上干。
The trouble is(that) he is ill. 糟糕的
表語從句用法拓展：
名詞性從句在be等系動(dòng)詞后作表語時(shí)被稱為表語從句，表語從句只能置于主句之后，而主句的動(dòng)詞只能是聯(lián)系動(dòng)詞。
例如：The problem is how we can get the things we need. 問題是我們?cè)鯓幽芘轿覀冃枰臇|西。(how在表語從句中充當(dāng)方式狀語)
The scissors are not what I need. 這把剪刀不是我所需要的。(what在表語從句中充當(dāng)賓語)
What I told him was that I would find him a good play.
我告訴他的是我會(huì)給他找個(gè)好劇本。(what在主語從句中作直接賓語，that作為表語從句的引導(dǎo)詞在該表語從句中不充當(dāng)句子成分，不能省略)
That is what I want to tell you. 那就是我想要對(duì)你講的。(what在表語從句中充當(dāng)直接賓語)
That is why she failed to pass the exam. 那就是她考試不及格的原因。(why在表語從句中充當(dāng)原因狀語)
注意：“That is why...”是常用句型，意為“這就是……的原因/因此……”，其中why引導(dǎo)的名詞性從句在句中作表語，該句型通常用于針對(duì)前面已經(jīng)說明過的原因進(jìn)行總結(jié)：
如：That is why you see this old woman before you know, Jeanne.珍妮，這就是現(xiàn)在這個(gè)老太婆出現(xiàn)在你面前的原因。(前文提到Jeanne對(duì)老婦人顯得蒼老憔悴深感詫異，說話人對(duì)她講述了其中的原因之后，用這一句來進(jìn)行概括)。//ThatiswhyIcame.這就是我來的原因。下面是兩個(gè)與“That is why...”形式相似的結(jié)構(gòu)，它們與“That is why...”結(jié)構(gòu)之間的關(guān)系要能夠辨析清楚：
�。�1）“That is why...”與“That is the reason why...”同義，只不過從語法結(jié)構(gòu)上講，“That is the reason why...”中why引導(dǎo)的是—個(gè)定語從句，將其中的the reason去掉則與“That is why...”結(jié)構(gòu)一樣：
如：That is(the reason) why I can not agree. 這就是我不能同意的理由。
�。�2）“That is because...”句型中從屬連詞because引導(dǎo)的名詞性從句在此作表語，這也是個(gè)常用句型，意為“這就是為什么……/因?yàn)椤��！癟hat is because...”與“That is why...”之間的不同在于“That is because...”指原因或理由，“That is why...”則指由于各種原因所造成的后果：
如：He did not see the film last night.
That is because he had to help his little sister with her homework. 昨天晚上他沒有去看電影，那是因?yàn)樗脦椭拿妹米鲎鳂I(yè)。(第一句話說明結(jié)果，第二句話說明原因)
He had seen the film before. That is why he did not see it last night. 他以前曾看過那部電影，因此他昨天晚上沒有去看。(第一句話說明原因，第二句話說明結(jié)果)