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。