Student or Learner
My book says "I don't know why to go there." is ungrammatical.
I don't know the reason.
"What to do, where to go, when to start" are all grammatical.
I don't know the differences.
I am looking forward to your kind explanations.
"What to do, where to go, when to start" are not complete sentences. They can be termed as phrases with a pronoun (what, when, where) as head of the phrase. So they can act as subject or object of a sentence. Where as why is not a pronoun and is normally used as an adverb or a conjunction. You can feel the difference by using them in sentence.
I donít know what to do after the meeting. (It expresses a definite meaning or thought)
I donít know where to go after the meeting. (A complete thought)
I donít know when to start work after the meeting. (A complete thought)
Now if you say:
I donít know why to go after the meeting. (What is the thought?)
It does not work and that is the reason.