一．定义：
表语从句就是用一个句子作为表语。
位于系动词之后，说明主语身份，特征，属性或状态。
比较：
The problem is puzzling.
主语连系动词形容词作表语
The problem is when we can get a pay rise.
主语连系动词一个句子作表语---表语从句
二、系动词的分类（了解哪些词为系动词）
系动词有表示状态的；有表示状态变化的。
表示状态的有：
1）be, seem ,appear 等。
2）look ,sound ,feel ,taste ,smell 等，由感官动词变化而来。
意思为“看（听、摸、尝、闻）起来”。
Mr. Li looks quite young. 李先生看起来很年轻。
3）stand ,keep ,prove ,remain 等由不及物动词转化而来。
The machine has been standing idle. 这台机器一直闲着不用。
2.表示状态变化的：
become ,get ,grow, fall, turn等
He has fallen ill. 他已经病倒了。
三连接表语从句与主句的关系词主要有三类：
1. 从属连词：that（无有词义）, whether（是否,if （是否）
2. 关系代词：who（谁，主格）, whom（谁，宾格）,whose谁的，所有格）, what（什么）,which（哪个，哪些）
3. 关系副词：when（什么时候）where（什么地方）why（为什么）, how（怎样）例如：
The fact is that we have lost the game. 事实是我们已经输了这场比赛。
That’s just what I want. 这正是我想要的。
This is where our problem lies. 这就是我们的问题所在。
That is why he didn’t come to the meeting.那就是他为什么不到会的原因。
It looks as if it is going to rain. 看上去天要下雨了。
It is because you eat too much那是因为你吃得太多了。
The reason why he was late was that he missed the train by one minute this morning
（当主语是reason时，表语从句要用that引导而不是because）
另外，与其他名词从句有所不同的是as, as if/though, because也可用来引导表语从句.如：
He looked just as he had looked ten years before.他看起来还与十年前一样。
She seems as if she had done a great thing.她看起来好像做了一件大事。
It is because you eat too much.那是因为你吃得太多了。
【注意】because可引导表语从句，但与之同义的since, as, for等也不用于引导表语从句。
四注意事项（重点看）
1. 表语从句要用陈述语序。
如：
That is where the famous scientist was born.
This is why she is so happy today
2. that和what在引导的表语从句
That本身没有词义，在句中只起连接作用，不充当句子成分；
what则表示“所……的（人或事）”，在表语从句中充当主语或宾等。
如：
That fact is that more than seventy percent of the earth’s surface is covered by water.
What he told you was what had been discussed at the meeting.
3. If 与whether
均意为“是否”引导表语从句时，只能用whether, 不能用if。
如：
The question is whether we can finish our work by tomorrow evening.
4 语气
1）主语为advice, order, idea, notion, proposal, suggestion, request等一些表示“建议、劝说、命令”的名词，之后的表语从句中，谓语动词用虚拟语气。
should+动词原形表示，should 可省略。
如：
My suggestion is that we (should) start early tomorrow. 我的建议是我们明天一早就出发。
Our only request is that this should be settled as soon as possible.
我们唯一的请求就是尽快解决这个问题。
2）As if/though引导的表语从句时，从句谓语多用虚拟语气。
这是因为从句中的情况与事实不相符。
具体来说，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“had+done”，
如果从句表示将来的可能性不大，用would(might ,could )+动词原形，如： Li Lei is now in a new jacket he looks as if he were an American boy.
The girl is giving us a vivid description of moon. It seems as if she had been to the moon many times.
但是，如果as if ,as 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，如：The clouds are gathering. It looks as if is going to rain.
5 时态
不像宾语从句，在有表语从句的复合句中，主句时态和从句时态可以不一致。
The question is who will travel with me to Beijing tomorrow.
The question is why he cried yesterday.
6 记住两个句型
A．The reason is that…B．That is why （because）…
例如：
The reason is that he didn't catch the bus.理由是他没有赶上公共汽车。
That is why he came late.那就是他晚的原因。
That is because he didn't catch the bus.那是因为他没有赶上公共汽车。
