Is "When I ask then you answer." not correct?
It was marked as correct for me
What purpose does 'thì' have in this sentence? Does "Khi tôi hỏi bạn trả lời?" not also mean the same thing?
it's just a sentence structure (khi...thì...) that differs from English
Is the 'thi' necessary?
more natural with it than without it? I'd say yes.
why? because of the flow, the rhythm. it's also like talking in English without conjunctions to link clauses.
Can you substitute 'khi nào' above?
When i ask, you answer
'When I (capitalized) ask, (then) you answer' both are correct and accepted