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。

　　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.

　　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。