what to do并不是一個句子,而只是一個名詞性成分,一般稱為 “疑問詞 + 不定式” 結(jié)構(gòu).
所以也就不能用句子的語法來分析該結(jié)構(gòu)了.
不過如果放在具體句中時,可以換寫成相應的名詞性從句.
如：I don't know what to do.
= I don't know what I should do.
如果疑問詞為代詞時,則要求該疑問詞必須做不定式中某個動詞或介詞的賓語.
再如：I don't know whom to send this letter to.
= I don't know whom I should send this letter to.
我不知道該把這封信送給誰.
此例句中,whom充當介詞to的賓語,即：
whom to send this letter to
= to send this letter to whom