Question

what to do的用法？

Answer 1

what to do意为做什么、怎么办，问的是动作的对象，而what是代词在句中中做宾语，所以what to do中不需要再加宾语。
例句有：
1、I don't quite know what to do next.
我不太清楚下一步该怎么办。

2、He was considering what to do next.
他在考虑下一步怎么办。

3、I just don't know what to do for the best.
我就是弄不清做什么才好。

4、She never had the least idea what to do about it.
这事怎么办，她一点主意都没有。

5、I don't know what to do with all the food that's left over.
我不知道怎样处理所有这些剩饭剩菜。

Answer 2

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