1.定义：用作表语的从句叫做表语从句。
2.构成：关联词+简单句
3.引导表语从句的关联词的种类：
（1）从属连词that.如：
The trouble is that I have lost his address.麻烦是我把他的地址丢了。
（2）从属连词whether，as，as if.如：
He looked just as he had looked ten years before.他看起来还与十年前一样。
The question is whether they will be able to help us.问题是他们是否能帮我们。
注：从属连词if一般不用来引导表语从句，但as if却可引导表语从句，如：
All this was over twenty years ago，but it’s as if it was only yesterday.
这都是20多年前的事了，但宛如昨天一样。
能跟表语从句的谓语动词一般为系动词be，seem，look等。如：
It looked as if it was going to rain.看起来天要下雨了。
（3）连接代词who，whom，whose，what，which，whoever，whatever，whichever
连接副词 where，when，how，why.
如：The problem is who we can get to replace her.问题是我们能找到谁去替换她呢。
The question is how he did it.问题是他是如何做此事的。
That was what she did this morning on reaching the attic.那就是她今晨上了阁楼干的。
解释：
1.连词because可引导表语从句。如：
I think it is because you are doing too much.我想这是因为你做得太多。
2.在一些表示“建议、劝说、命令”的名词后面的表语从句中，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。
2）主语从句
1.定义：用作主语的从句叫做主语从句。
2.构成：关联词+简单句
3.引导主语从句的关联词有三类：
（1）从属连词that.如：That they were in truth sisters was clear from the facial resemblance between them.
很明显，她们确是亲姐妹，她们的脸型很相似。
（2）从属连词whether.如：
Whether he’ll come here isn’t clear.他是否会来这里还不清楚。
（3）连接代词who，whom，whose，what，which，whoever，whatever，whichever
连接副词 where，when，how，why.如：
What she did is not yet known.她干了什么尚不清楚。
How this happened is not clear to anyone.这事怎样发生的，谁也不清楚。
Whoever comes is welcome.不论谁来都欢迎。
Wherever you are is my home —— my only home.你所在的任何地方就是我的家——我唯一的家。
解释：
1.主语从句能用it作形式上的主语。常以it作形式主语的句型有：
A.It+be+形容词（obvious，true，natural，surprising，good，wonderful，funny，possible，likely，certain，probable，etc.）+that从句。如：
It is certain that she will do well in her exam.毫无疑问她考试成绩会很好。
It is probable that he told her everything.很可能他把一切都告诉她了。
B.It+be+名词词组（no wonder，an honor，a good thing，a pity，no surprise，etc.）+that从句。如：
It’s a pity that we can’t go.很遗憾我们不能去。
It’s no surprise that our team should have won the game.我们没赢这场比赛真意外。
C.It+be+过去分词（said，reported，thought，expected，decided，announced，arranged，etc.）+that从句。如：
It is said that Mr.Green has arrived in Beijing.据说格林先生已经到了北京。
It is reported that China has sent another man-made earth satellite into orbit.
据报道中国又成功地发射了一颗人造地球卫星。
D.It+seem，happen等不及物动词及短语+that从句。如：
It seems that Alice is not coming to the party at all.Alice似乎不来参加晚会。
It happened that I was out that day.碰巧我那天外出了。
E.It+doesn’t matter（makes no difference，etc.）+连接代词或连接副词引起的主语从句。如：
It doesn’t matter whether she will come or not.她是否来这无关紧要。
It makes no difference where we shall have the meeting.我们在哪里开会毫无区别。
F.当that引导的主语从句出现在疑问句中时，要以it作形式主语，而把主语从句后置。如：
Is it true that the scientist will give us a lecture next week？ 下周那位科学家将给我们作报告是真的吗？
Does it matter much that they will not come tomorrow？ 他们明天不来很要紧吗？
G.当主语从句出现在感叹句中时，要以it作形式主语，而把主语从句后置。如：
How strange it is that the children are so quiet！ 孩子们这么安静真奇怪！
2.注意连接代词whoever，whatever，whichever等引导主语从句的含义。
Whoever comes will be welcome.（whoever=the person who）来的人将受到欢迎。
Whatever he did was right.（whatever=the thing that）他所做的事情是正确的。
Whichever of you comes in will receive a prize.（whichever=anyone of you who）你们当中不论哪个进来将会得到奖
