thank you so much .

When is a 'generalization' of 'what time' and allows a more flexible answer; moreover it usually doe not want a 'too precise' answer. Example: q1 -- when did you study German ? a1 -- in my childhood. a2 -- between 2 pm of Jan 3 1990 and 8 pm of Dec 20 1993 (a1) and (a2) say the same thing, but (a2) contains a precise 'what time' answer, but usually this level of detail is not required by a question of the kind 'when' ?

When is related to time. for example: When are you leaving? in the afternoon what time is related to time of clock. for example : what time are you leaving? at 5 P.M

It depends on the sentence. In sentences they're interchangeable like this one: What time are you going to the store? When are you going to the store? In others you must use when. Especially when you're speaking in past tense. When did you buy that? (You can't use what time) When was the last time the Dallas Cowboys won a Super Bowl?