【英語語法分類彙總】表語從句
一、定義：
A. 表語從句就是用一個句子作爲表語。說明主語是什麼或者怎麼樣，由名詞、形容詞或相當於名詞或形容詞的詞或短語充當，和連繫動詞一起構成謂語。
例句：The problem is when we can get a pay rise. 問題是，當我們可以得到加薪。
B. 連接表語從句的連接詞有：that, what, who, when, where, which, why。例句：
He has become what he wanted to be ten years ago.
他已經成爲了他10年前想成爲的。
She has remained where I stood yesterday for an hour.
她一直在我昨天站的地方站了一個小時。
His suggestion is that we should stay calm.
他的建議是，我們應該保持冷靜。
The question is when he can arrive at the hotel.
問題是，他什麼時候可以到達酒店。
二、注意：
A. 表語從句一定要用陳述語序。
False: The question is when can he arrive at the hotel.
Right: The question is when he can arrive at the hotel.
B. 不可以用if，而用whether連接表語從句（as if 例外）；
引導賓語從句時可以互換if/whether，位於介詞後要用whether；
位於句首時要用whether；
引導表語從句，主語從句，同位語從句時要用whether。
False: The question is if the enemy is marching towards us.
Right: The question is whether the enemy is marching towards us.
Right: It looked as if he had understood this question.
C. 不像賓語從句，在有表語從句的複合句中，主句時態和從句時態可以不一致。
Right: The question is who will travel with me to Beijing tomorrow.
Right: The question is why he cried yesterday.
D. that在表語從句中不可以省掉。
表語從句的基本用法
表語從句只能置於主句之後，而主句的動詞只能是聯繫動詞。
名詞性從句在be等系動詞後作表語時被稱爲表語從句，例如：
The problem is how we can get the things we need. 問題是我們怎樣能弄到我們需要的東西。(how 在表語從句中充當方式狀語)
The scissors are not what I need. 這把剪刀不是我所需要的。(what在表語從句中充當賓語)
What I told him was that I would find him a good play. 我告訴他的是我會給他找個好劇本。(what在主語從句中作直接賓語，that作爲表語從句的引導詞在該表語從句中不充當句子成分，不能省略)
That is what I want to tell you. 那就是我想要對你講的。(what在表語從句中充當直接賓語)
That is why she failed to pass the exam. 那就是她考試不及格的原因。(why在表語從句中充當原因狀語)
注意：“That is why...”是常用句型，意爲“這就是……的原因/因此……”，其中why引導的名詞性從句在句中作表語，該句型通常用於針對前面已經說明過的原因進行總結，又如：That is why you see this old woman before you know, Jeanne. 珍妮，這就是現在這個老太婆出現在你面前的原因。(前文提到Jeanne對老婦人顯得蒼老憔悴深感詫異，說話人對她講述了其中的原因之後，用這一句來進行概括)。
下面是兩個與“That is why...”形式相似的結構，它們與“That is why...”結構之間的關係要能夠辨析清楚：
(1)“That is why...”與“That is the reason why...”同義，只不過從語法結構上講，“That is the reason why...”中why引導的是—個定語從句，將其中的the reason去掉則與“That is why...”結構一樣，例如：
That is (the reason) why I cannot agree. 這就是我不能同意的理由。
(2)“That is because...”句型中從屬連詞because引導的名詞性從句在此作表語，這也是個常用句型，意爲“這就是爲什麼……/因爲……”。“That is because...”與“That is why...”之間的不同在於“That is because...”指原因或理由，“That is why...”則指由於各種原因所造成的後果， 例如：
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，那是因爲他得幫助他的妹妹做作業。(第一句話說明結果， 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，第二句話說明結果)
