第二類，用連接代詞who、whose、which、what，連接副詞when、where、how、why，以及連詞whether（或if）引導(dǎo)的主語從句，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.

從以上例句可以看出，這類主語從句可以直接放在句首作主語，也可以使用先行it結(jié)構(gòu)，把主語從句放在后面，兩種結(jié)構(gòu)可以互換，意義上無差異，但用if 引導(dǎo)主語從句時(shí)，只能采取先行it結(jié)構(gòu)，也就是說if不可以引導(dǎo)置于句首的主語從句。