表语从句的虚拟语气 在表示建议、 劝告、命令含义的名词作主句的主语时，表语从句的谓语动词需用"should＋动词原形"表示，should可省略。常见的词有：advice，suggestion，order，proposal，plan，idea，request，requirement等。

My suggestion is that we(should)start early tomorrow.

我的建议是明天早点出发。 because，why引导的表语从句 That's because he didn't understand me.(That's because...强调原因)

That's why he got angry with me.(That's why...强调结果) 　　2.应注意的问题

　　(1)as if/though引导的表语从句常置于系动词look，seem，sound，be，become等后面，常用虚拟语气，即形式常用过去式的某种时态。

　　The elephant feels as if/though it were a wall.

　　大象摸起来像一堵墙。

　　(2)what引导的主语从句表示结果或名词reason作主语时，后面的表语从句表示原因时要用that引导，不宜用because。

　　What cost him his life was that he was too careless.

　　使他丧命的原因是他太粗心了。

　　The reason why I was sad/for my being sad was that he didn't understand me.

　　我难过的原因是他不理解我。

　　(3)连接词that引导表语从句时，that不能省略，尤其是从句较长或结构较复杂时。

The problem with looking into space from the earth is that there is lots of dust in the earth's air.