why to infinitive

Hello, I am a Japanese who is learning English at a school. This Monday I have been taught something about usage of the word "why". Our teacher taught us that we can say "what to do", "who to do", "which one to do", "where to do" or "when to do", but we can never say "why to do".
What he said is like this;
@@ @ I donft know what to do now. (OK)
@@ Donft you know which book to read next? (OK)
@@ I donft know who to vote for. (OK)
@@I donft know where to go today. (OK)
@@I donft know when to go there. (OK)
<BUT> I donft know why to go there (Wrong!)
At the lecture I asked him the reason why we should not use gwhy to infinitiveh but he said he himself did not know it. So I am asking people here to explain grammatically or semantically why we should not use gwhy to infinitiveh. Any comments on this subject are welcome. Thank you.

Ugh! Something wrong happened. I've to write again the message I posted.


Hello, I am a Japanese who is learning English at a school. This Monday I have been taught something about usage of the word "why". Our teacher taught us that we can say "what to do", "who to do", "which one to do", "where to do" or "when to do", but we can never say "why to do".
What he said is like this;
I don't know what to do now. (OK)
Don't you know which book to read next? (OK)
I don't know who to vote for. (OK)
I don't know where to go today. (OK)
I don't know when to go there. (OK)
<BUT> I don't know why to go there (Wrong!)
At the lecture I asked him the reason why we should not use “why to infinitive” but he said he himself did not know it. So I am asking people here to explain grammatically or semantically why we should not use “why to infinitive”. Any comments on this subject are welcome. Thank you.

After a rather thorough search of the net, I couldn't find any information on the subject. That last sentence should read something like: "I don't know why I should go there." "I don't know why I have to go there." "Why should I go there?" ....or in the colloquial realm: "Why go there?"...etc. etc. The possible combinations are endless.

You will sometimes see "why + infinitive" in advertisements or articles when giving a reason to do something.

For example:

"How and why to do your own landscaping."

"Why to choose John Doe as your next representative."

"Why to consider a career in plumbing."

These are not the most elegant choices in terms of wording. It would sound better to say: "Why one might consider a career in plumbing." or "Why should you/one consider a career in plumbing." etc.

While the examples I gave may lack elegance, one will encounter them from time to time.