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(三)表语从句
表语从句放在连系动词之后，充当复合句中的表语。
A 可接表语从句的连系动词
可接表语从句的连系动词有be, look, seem, sound, appear等。
China is no longer what she used to be.
今日的中国不再是过去的中国了。
The question remains25 whether they will be able to help us.
问题还是他们能否帮我们。
At that time, it seemed as if I couldn't think of the right word anyhow.
当时，我似乎怎么也想不出一个恰当的字眼来。
B as, as if / though引导的表语从句
He looked just as he had looked ten years before.
他看起来还与十年前一样。
It sounds as if someone is knocking at the door.
听起来好像有人在敲门。
C because, why引导的表语从句
That's because he didn't understand me.
那是因为他没有理解我。 (That's because...强调原因)
That's why he got angry with me.
那正是他对我生气的原因。 (That's why...强调结果)
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。
The reason why I was sad was that he didn't understand me.
我难过的原因是他没有理解我。
D 使用虚拟语气的表语从句
在表示建议、劝告、命令含义的名词后的表语从句，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, proposal26, 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。
His proposal was that they (should) challenge27 the other classes to a friendly competition.
他的提议是挑战别的班级进行一场友谊竞赛。
From:https://www.zgghmh.com/CET-4/20000725.asp