一、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, 例如:
1.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(dāng)方式狀語)
2.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)
3.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)
4.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(dāng)直接賓語)
5.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)

二、在表語從句中,“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前面已經(jīng)說明過的原因進(jìn)行總結(jié),例如:
1. 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后,用這一句來進(jìn)行概括)。
2. That is why I came.
這就是我來的原因。

三、“That is why...”的形式相似結(jié)構(gòu)有以下兩種,
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, 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(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, 例如:
1)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)
2)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)