表语：说明主语的性质、特征、状态或身份，说明主语是什么或者怎么样。总之表语是对主语的解释和说明，是主语具体化，并且常与连系动词一起使用。
表语从句：在复合句中作表语的从句, 就叫做表语从句。表语从句一般放在系动词之后,结构是“主语+系动词+表语从句”。
★连系动词：be动词、表示持续的系动词(keep, remain, stay)、感官动词feel(摸起来，感觉) , smell(闻起来) , sound(听起来) , taste(尝起来，吃起来) 等、表示变化的系动词(become, grow, turn, fall, get, go,)、表终止的系动词prove, turn out(结果是，证明是)、seem, appear(看起来……)
连系动词通常不用于被动语态和进行时态中。
He has become what he wanted to be ten years ago.
他已经成为了他10年前想成为的样子。
His suggestion is that we should stay calm.
他的建议是，我们应该保持冷静。
1.表语从句的引导词
①从属连词
-学东慧9160智技元学科件4add软有公网途广限升优司 that / whether /as if /as though/as/because
(1) that引导表语从句本身没有词义，在句中只起连接作用，不充当句子成分，一般不能省略。
That fact is that more than seventy percent of the earth’s surface is covered by water.
连接词that一般不能省略，但当主句中含动词do的某种形式时，that可以省略。
What I want to do is (that) I can go up to him and thank him. 我想做的事是走到身边去感谢他。
(2) whether引导表语从句表示“是否”,但不充当句子的成分。
The question is whether we can finish our work by tomorrow evening.
if 与whether均意为“是否”，但引导表语从句时，只能用whether, 不能用if。
(3)as if/though“好像”，引导表语从句时要注意语态。
如果句中的情况与事实不相符，从句多用虚拟语气。如果从句表示与现在事实相反,谓语动词用一般过去式;如果从句表示与过去事实相反时,谓语动词要用“had + 过去分词 ”，如果从句表示将来的可能性不大，用would (might ,could )+动词原形。
Li Lei is now in a new jacket. He looks as if he were an American boy. (现在事实相反)
The girl is giving us a vivid description of the moon. It seems as if she had been to the moon many times. (与过去事实相反)
It looks as if it might rain. (与将来事实相反)
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但是，如果as if/though引导的表语从句所表示的与事实相符，从句则用陈述语气。
The clouds are gathering. It looks as if is going to rain.
(4) as引导表语从句
He looked just as he had looked ten years before.
他看起来还与十年前一样。
(5) because引导表语从句
常用结构：This/That/It is/was because...
That is because I don’t like Chinese.
②连接代词
who/whom/whose/what/which/whoever/whatever/whichever/whomever，在表语从句中做主语、宾语、表语和定语。
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Tom is no longer what he used to be. (what做表语)
The problem is who is fit for this job. (who做主语)
This is what I want to tell you. (what做宾语)
The problem is whose work is the best. (whose做定语)
③连接副词
when/where/how/why，在表语从句中做状语。
The question is how he did it. 问题是他是如何做此事的。
The question is where we can live.问题是我们能住在哪儿。
2.特例强调
(1)reason做主语或主语中包含事件的起因时,后面的表语从句表示原因时要用that来引导,而不用because;why引导主语从句做主语时，表语从句用that引导，不用because。
The reason is that he got up late.
Why he is late is that he got up late.
(2)that is why/because
①That is why ...是常用句型,其中why引导的从句在句中作表语,该句型通常用于针对前面已经说过的原因进行总结。That is the reason why ...与That is why ...是同义的，“这就是……的原因/因此……”，但是从语法结构上讲That is the reason why ...中的why引导的是一个定语从句。
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)
That is the reason why she failed to pass the exam.
那就是她考试不及格的原因。(why 在定语从句中充当原因状语)
②That 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“这就是因为……”。
That is because I got up late. 这是因为我起床迟了。
③“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。
I was angry. That was because he didn’t understand me.我生气是因为他不理解我。(表语从句强调原因)
That’s why he got fired from that firm.那正是他被公司解雇的原因。(表语从句强调结果)
(3)The reason (why.../for...)is /was that... “...的原因是...”
The reason for his absence is that he got up late.
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The reason why he is absent is that he got up late.
他缺席的原因是他起床迟了。
(4)使用虚拟语气的表语从句
在表示建议、劝告、命令、计划含义的名词后的表语从句，谓语动词需用“should+动词原形”表示虚拟语气，should可省略。常见的词有：advice，suggestion，order，request，proposal，plan，idea等。
My suggestion is that we (should) start early tomorrow.我的建议是我们明天一早就出发。
来源：雅途Online
编辑：小徐