分类
名词性从句：主语从句、表语从句、宾语从句和同位语从句。
形容词性从句：定语从句
副词性从句：时间状语从句、原因状语从句、地点状语从句、条件状语从句、让步状语从句、目的状语从句、结果状语从句和方式状语从句。
名词性从句
单纯连接词：that, if, whether(句中不充当成分)
疑问代词：who, whoever, whichever, what, whatever
疑问副词：when, where, why, how
主语从句
That we shall be late is certain.他迟到是肯定的。
Who he is doesn’t concern me.
What she did is not yet known.
How the prisoner escaped is a complete mystery.
It is certain that we shall be late.
It’s not yet known what she did.
重要考点
That 不充当成分但不可省略。
为保句子平衡，可用it 做形式主语，真正主语从句放在句末.
That the earth goes around the sun is well-known to everybody.地球围绕着太阳转是众所周知的事实。(= It is well-known to everybody that the earth goes around the sun.)
谓语动词单数
只能使用whether表示是否, if 不可以使用。
用何连词看从句中缺少什么成分来决定
表语从句
The problem is who we can get to replace her.
That is what he meant.
The question is how he did it.
That’s where he lives.
This was when I really got to know the United States.
Things are not always as they seem to be.
His first question was whether Tom had arrived yet.
The problem is that he does not come.
重要考点
That 一般不可以省略
只能使用whether表示是否, if 不可以;但as if 可以
The question is whether they will be able to help us. 问题是他们是否能帮我们。
All this was over twenty years ago, but it seems as if it was only yesterday.所有这一切过去20多年了，但似乎刚刚是昨天发生的一样。
用何连词看从句中缺少什么成分来决定