连接词：that,whether,if 不充当从句的任何成分）
连接代词：what, whatever, who, whoever, whom,
whose, which.
连接副词：when, where, how, why
不可省略的连词：
1. 介词后的连词
2. 引导主语从句和同位语从句的连词不可省略。
That she was chosen made us very happy.
We heard the news that our team had won.
比较：whether与if 均为"是否"的意思。 但在下列情况下，whether 不能被if 取代：
1. whether引导主语从句并在句首
2. 引导表语从句
3. whether从句作介词宾语
4. 从句后有"or not"
Whether he will come is not clear.
大部分连接词引导的主语从句都可以置于句末，用 it充当形式主语。
It is not important who will go.
It is still unknown which team will win the match.
