what to do的语法分析

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