表语从句作为英语中的一项重要从句，其内含的语法知识非常丰富，我们要掌握精髓，避开雷区，在使用时要格外注意以下几点：

一．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.

that is why/because

(1) That 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)

(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)

三. 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。

四. 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。

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