"When does he come?"

Translation: Wann kommt er?

[Kit845299]

Wann? = When? (asking for a time)
wenn = if/when (expressing a condition)

[stepintime]

...but "wenn / when" is also used to specify a time in a relative clause: "Wenn die Ampel grün wird, gehe ich über die Straße." "When (= once) the traffic light turns green, I cross the road."

I think it's a common mistake for learners to use "wann" in this kind of relative clause where you need to use "wenn". I suspect that in your (@Sindhu433685) question it's the same mistake the other way round. --> "wann?" = "when?" (question); vs. "..., wenn" = "... when" (relative clause).

You can't necessarily tell if "wenn" means "if" (condition) or "when" (time): "Wenn er kommt, gehe ich!" = "If he comes, I'll leave!" or "When he arrives, I'll leave!"

[jaehorse]

Is there any particular way to understand when to use 'wann' as opposed to 'wenn'?