在复合句中，作主语的从句称为主语从句。
一、主语从句的引导词及其在句中的功能
(一)连接词that和whether
[观察例句]
1．That she could come to help us made us very happy.
2．That the big earthquake had killed many people frightened people all over the world.
3．Whether we go by train or by boat makes no difference.
4．It is still unknown whether the plan will be carried out.
[归纳用法]
1．that引导主语从句，在从句中不充当任何成分，仅起连接作用，但不能省略。
2．whether引导主语从句，在从句中不充当任何成分，起连接作用，但有自己的意义，不可省略。
[名师点津]
whether引导的主语从句既可放在句首，也可放在句尾(前面需用it作形式主语)。
