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。

难点释疑：

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 honour，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. 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励。