I believe the difference should become clear to a beginner of Hindi in the initial stage of learning.
In English WHEN is used as and interrogative as well as a conjunctive adverb. The intonation of the speaker or the question mark in the written text along with the context make things clear.
In fact this sort of difference you could not in respect of adverbs WHERE, HOW and pronouns WHO, WHOM, WHICH, WHOSE. (These are which come to my mind, there may be more.) In all these cases the क-word (starting with k) is used in questions and ज-word (starting with j) in other places. Be careful that the question could be in an indirect speech as well (as in: He asked me when I would return = उसने मुझसे पूछा कि मैं कब लौटूंगा = usane mujhase poochaa ki maiN kab lauTooNgaa.)
I hope this much serves some purpose.
I am indeed sorry. I had typed on my laptop the text appearing herein as a contribution, and it seems I inadvertently got that pasted in a wrong place. It was actually meant for Peggy Peterson (her mail, today, 07:59). I should have been somewhat careful to check that I was not opening a wrong post to offer a response. Beg evryone's pardon. -- Yogendra