I don't know what to do what在其中做什么语 我一直以为What只在疑问句中出现 而且是句首

what to do 此处相当于what I should do.what做do的宾语
特殊疑问词+动词不定式往往可改成特殊疑问句,如：
I don't know what to say/how to do it.
I don't know what I　can　　say／how　I　should　do　it．