第二类，用连接代词who、whose、which、what，连接副词when、where、how、why，以及连词whether（或if）引导的主语从句，例如：

Who was responsible for the accident is not yet clear.

→ It is not yet clear who was responsible for the accident.

What he did is not yet known.

→ It is not yet known what he did.

Whether it is true remains a problem.

→ It remains a problem whether / if it is true.

从以上例句可以看出，这类主语从句可以直接放在句首作主语，也可以使用先行it结构，把主语从句放在后面，两种结构可以互换，意义上无差异，但用if 引导主语从句时，只能采取先行it结构，也就是说if不可以引导置于句首的主语从句。