主语从句就是在复合句中作主语的从句。主语从句常用that, whether, who, whom, whose, what, which, when, where, how, why, whatever, whoever等引导。
比如：
That she survived the accident is a miracle. （that 只是引导词，没有实际意义）
whoever makes mistake must correct them.
What you need is more practice.
Where we should live is a problem.
It is common knowledge that the whale is not a fish. （it 为形式主语，后that 引导的句子做句子正真的主语）
部分句子引用
http://www.yygrammar.com/ 感谢！