what to do并不是一个句子,而只是一个名词性成分,一般称为 “疑问词 + 不定式” 结构.
所以也就不能用句子的语法来分析该结构了.
不过如果放在具体句中时,可以换写成相应的名词性从句.
如：I don't know what to do.
= I don't know what I should do.
如果疑问词为代词时,则要求该疑问词必须做不定式中某个动词或介词的宾语.
再如：I don't know whom to send this letter to.
= I don't know whom I should send this letter to.
我不知道该把这封信送给谁.
此例句中,whom充当介词to的宾语,即：
whom to send this letter to
= to send this letter to whom