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表语从句用法详解
1. 表语从句只能置于主句后，主句的动词只能是连系动词。名词性从句在系动词后作表语时被称为表语从句。如：
① 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)
② 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)
2. 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。
辨析：
1)"That is why...”与“That is the reason why...”同义，但“That is the reason why...”中why引导的是定语从句，例如：
That is (the reason) why I cannot agree. 这就是我不能同意的理由。
(2)“That is because...”中从属连词because引导的名词性从句在此作表语，意为“这就是为什么……/因为……”。
“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， 例如：
He did not see the film last night. That is because he had to help his little sister with her homework. 昨天晚上他没有去看电影，那是因为他得帮助他的妹妹做作业。
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。
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because 和 why 引导的表语从句有什么不同
引导名词性从句的关联词除that外，还有who, whom, whose, what, which, whoever, whomever, whichever, whatever, when, where, why, how, if和whether等。
就你所提的why和because引导表语从句时的区别。
在句型That / It is + 表语从句中，why 表示“结果”，because 表示“原因”。如:
There was something wrong with his car. That was why he came late.
The boss scolded him. That was because he came late.
表语从句中what和who的用法
导语： 表语从句在一个 复合句中，从句作的是主句的表语成分。下面我为您收集整理了英语表语从句的用法，希望对您有帮助!
表语从句的定义：
A. 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。
例句：The problem is when we can get a pay rise. 问题是，当我们可以得到加薪。
B. 连接表语从句的连接词有：that, what, who, when, where, which, why。例句：
He has become what he wanted to be ten years ago.
他已经成为了他10年前想成为的。
She has remained where I stood yesterday for an hour.
她一直在我昨天站的地方站了一个小时。
His suggestion is that we should stay calm.
他的建议是，我们应该保持冷静。
The question is when he can arrive at the hotel.
问题是，他什么时候可以到达酒店。
表语从句的.注意事项：
A. 表语从句一定要用陈述语序。
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连接表语从句(as if 例外);
引导宾语从句时可以互换if/whether，位于介词后要用whether;
位于句首时要用whether;
引导表语从句，主语从句，同位语从句时要用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. 不像宾语从句，在有表语从句的复合句中，主句时态和从句时态可以不一致。
Right: The question is who will travel with me to Beijing tomorrow.
Right: The question is why he cried yesterday.
D. that在表语从句中不可以省掉。
表语从句的基本用法：
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。
名词性从句在be等系动词后作表语时被称为表语从句，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，又如：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)。
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，例如：
That is (the reason) why I cannot 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， 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)
表语从句中that和because的区别
虽然 why和 because 都可引导表语从句，但前者强调结果，后者强调原因。如：
The reason was that you don’t trust her. 原因是你不信任她。
The fact is that they are angry with each other. 事实是他们生彼此的气。
He was ill. That’s why he was sent to the hospital. 他病了，所以被送到医院来。
He was sent to the hospital. That’s because he was ill. 他被送到医院，是因为他生病了。
表语从句定义：表语从句（Predicative Clause）就是用一个句子作为表语。
表语（Predicative）的作用：说明主语是什么。可以由名词、形容词或相当于名词或形容词的词或短语充当表语的成分。
然而要注意，表语从句属于名词性从句，只是表语的其中一种情况——由名词充当表语。
常见的引导表语从句的从属连词（Subordinating Conjunction）有when，where，why，who，how，that。它们都有代词（Pronoun）词性，即具备名词（Noun）性质，所以可以引导主语从句、同位语从句、宾语从句、表语从句全部四种名词性从句。
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