Use of adjective to tell a purpose or its nature

Hi. I think we can put an adjective that tells something about the nature (could it be a purpose too?) of the noun that follows it. What is the difference?

I have a verb question.

I have a question on verbs.

I have two verb questions.

I have two questions on verbs.

I have three present perfect tense questions.

I have three questions on present perfect tenses.