if可以引導賓語從句，賓語從句若還原成簡單句是一般疑問句。還能引導條件狀語從句，讓步狀語從句。但if不能引導主語從句，補語從句，同位語從句和定語從句。

i don"t know if he will come here tonight.賓語從句 ----will he come here tonight?

i will tell him if he come here tonight.條件狀語從句

If he said that,he didn"t expect you to take it personally.讓步狀語從句。

what可以引導賓語從句，主語從句，補語從句和同位語從句，這些名詞性從句若還原成簡單句是特殊疑問句。還能引導失去先行詞的定語從句。

i don"t know what he was doing. 賓語從句---what was he doing?

what was he doing is a question.主語從句

the question is what he was doing.補語從句

what she says goes.失去先行詞的定語從句。這裡的what無疑問意義。