一、何謂“表語”?
說到表語從句，就不得不先說說什么是“表語”。
表語是英語區(qū)別于漢語所特有的一種詞類，是起表述作用的句子成分：
從形式上講，它是位于系動(dòng)詞之后的成分;
從含義上講，它是用來描述主語“是什么或是怎么樣”的成分。
表語是用來說明主語的身份、性質(zhì)、品性、特征和狀態(tài)的。表語必須和系動(dòng)詞一起構(gòu)成系表結(jié)構(gòu)，作為句子的復(fù)合謂語。表語通常由名詞、形容詞、介詞短語、不定式、形容詞化的現(xiàn)在分詞或過去分詞以及從句來充當(dāng)。
二、何謂“表語從句”?
當(dāng)句子的表語由從句來充當(dāng)時(shí)，這個(gè)從句就叫做表語從句。
表語從句一般都是位于系動(dòng)詞之后的。
三、表語從句的連接詞
常用來引導(dǎo)表語從句的連接詞主要有：
that
whether
what/who/whom/whose/which
when/where/why/how及其短語
(一)that
當(dāng)that引導(dǎo)表語從句時(shí)，只起連接作用，不在表語從句中充當(dāng)任何句子成分。如：
His advice is that we should leave for Shanghai before dawn.
Her plan is that she will call the police first and then try to stop the thief from running away.【注意】
當(dāng)名詞主語是表示建議、命令、要求等含義(如advice、order、idea、notion、proposal、suggestion、request等)時(shí)，其表語從句中的謂語動(dòng)詞一般要采用“should+do”的形式，should可以省略。如：
The officer's order was that all the soldiers (should) never surrender to the enemy.(二)whether
當(dāng)表語從句要表達(dá)“是否”的含義時(shí)，連接詞要用whether。如：
His question is whether they can arrive in London on time.需要注意的是，盡管if也可以表示“是否”的含義，但在表語從句中只能用whether，不能用if。
(三)what/who/whom/whose/which
這些疑問代詞如果用來連接表語從句，既起著連接作用，同時(shí)又在從句中充當(dāng)句子成分，可以作主語、賓語、定語等。如：
My concern is what he really expects to get from us.
Her question is who has broken the glass.
Our dilemma is which side we should take.
Their worry is whom she can rely on.
His puzzle is whose money it belongs to.(四)when/where/why/how及其短語
這些疑問副詞如果用來連接表語從句，既起著連接作用，同時(shí)又在從句中充當(dāng)句子成分，一般作狀語。如：
The question is when he will show up at the party tomorrow.
He remains where I was last night.
That's why he was late for school this morning.
Her concern is how he managed to climb into her kitchen yesterday.【注意】
當(dāng)主語是reason時(shí)，表語從句要用that連接，而不是because。如：
The reason why he was late was that he missed the first morning bus.
四、表語從句的特殊連接詞
除了以上常見的連接詞外，還有一些特殊的連接詞可以用來連接表語從句。
(一)(just) as
表示“正如”，經(jīng)常與表象系動(dòng)詞look/seem/appear搭配使用。如：
She looks just as she was ten years ago.(二)as if/though
表示“好像”，也常與表象系動(dòng)詞look/seem/appear搭配使用。如：
It looks as if/though it's going to rain soon.
【注意】
這種情況下，as if/though后面的表語從句中的謂語動(dòng)詞一般采用虛擬語氣形式，其用法是：該謂語動(dòng)詞的時(shí)態(tài)要采用相應(yīng)的過去時(shí)態(tài)。如：
She acts as if/though she were the hostess of this house.
They looked as if/though they had lived here for a long time.(三)because
前面講到why可以用來連接表語從句，其實(shí)because也可以用來連接表語從句。如：
She is very familiar with the film. This is because she has already seen the film.【注意】
在這種情況下，盡管since/for/as都可以表示原因，但這三個(gè)詞不能用來連接表語從句。
說到表語從句，就不得不先說說什么是“表語”。
表語是英語區(qū)別于漢語所特有的一種詞類，是起表述作用的句子成分：
從形式上講，它是位于系動(dòng)詞之后的成分;
從含義上講，它是用來描述主語“是什么或是怎么樣”的成分。
表語是用來說明主語的身份、性質(zhì)、品性、特征和狀態(tài)的。表語必須和系動(dòng)詞一起構(gòu)成系表結(jié)構(gòu)，作為句子的復(fù)合謂語。表語通常由名詞、形容詞、介詞短語、不定式、形容詞化的現(xiàn)在分詞或過去分詞以及從句來充當(dāng)。
二、何謂“表語從句”?
當(dāng)句子的表語由從句來充當(dāng)時(shí)，這個(gè)從句就叫做表語從句。
表語從句一般都是位于系動(dòng)詞之后的。
三、表語從句的連接詞
常用來引導(dǎo)表語從句的連接詞主要有：
that
whether
what/who/whom/whose/which
when/where/why/how及其短語
(一)that
當(dāng)that引導(dǎo)表語從句時(shí)，只起連接作用，不在表語從句中充當(dāng)任何句子成分。如：
His advice is that we should leave for Shanghai before dawn.
Her plan is that she will call the police first and then try to stop the thief from running away.【注意】
當(dāng)名詞主語是表示建議、命令、要求等含義(如advice、order、idea、notion、proposal、suggestion、request等)時(shí)，其表語從句中的謂語動(dòng)詞一般要采用“should+do”的形式，should可以省略。如：
The officer's order was that all the soldiers (should) never surrender to the enemy.(二)whether
當(dāng)表語從句要表達(dá)“是否”的含義時(shí)，連接詞要用whether。如：
His question is whether they can arrive in London on time.需要注意的是，盡管if也可以表示“是否”的含義，但在表語從句中只能用whether，不能用if。
(三)what/who/whom/whose/which
這些疑問代詞如果用來連接表語從句，既起著連接作用，同時(shí)又在從句中充當(dāng)句子成分，可以作主語、賓語、定語等。如：
My concern is what he really expects to get from us.
Her question is who has broken the glass.
Our dilemma is which side we should take.
Their worry is whom she can rely on.
His puzzle is whose money it belongs to.(四)when/where/why/how及其短語
這些疑問副詞如果用來連接表語從句，既起著連接作用，同時(shí)又在從句中充當(dāng)句子成分，一般作狀語。如：
The question is when he will show up at the party tomorrow.
He remains where I was last night.
That's why he was late for school this morning.
Her concern is how he managed to climb into her kitchen yesterday.【注意】
當(dāng)主語是reason時(shí)，表語從句要用that連接，而不是because。如：
The reason why he was late was that he missed the first morning bus.
四、表語從句的特殊連接詞
除了以上常見的連接詞外，還有一些特殊的連接詞可以用來連接表語從句。
(一)(just) as
表示“正如”，經(jīng)常與表象系動(dòng)詞look/seem/appear搭配使用。如：
She looks just as she was ten years ago.(二)as if/though
表示“好像”，也常與表象系動(dòng)詞look/seem/appear搭配使用。如：
It looks as if/though it's going to rain soon.
【注意】
這種情況下，as if/though后面的表語從句中的謂語動(dòng)詞一般采用虛擬語氣形式，其用法是：該謂語動(dòng)詞的時(shí)態(tài)要采用相應(yīng)的過去時(shí)態(tài)。如：
She acts as if/though she were the hostess of this house.
They looked as if/though they had lived here for a long time.(三)because
前面講到why可以用來連接表語從句，其實(shí)because也可以用來連接表語從句。如：
She is very familiar with the film. This is because she has already seen the film.【注意】
在這種情況下，盡管since/for/as都可以表示原因，但這三個(gè)詞不能用來連接表語從句。