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 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
It is reported that
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
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