Why does "in der Nacht" mean "that night"? Couldn't it also mean "in the night"?!
It can mean both "at night" (in general) and "that night" (on a specific night).
"In the night" might be technically acceptable and could have either meaning, but its ambiguity would probably make it less preferable to whichever meaning you intend.
Is it wrong in English to say, "at" the night?. For example, to say, "What did you drink at that night?"
Couldn't it rather refer to last night? That would make the sentence sensible. that night is totally out of context.
There is no context in Duolingo, but here one is readily imaginable:
"I don't know what happened at that party. When I woke up, I was in a swamp surrounded by grinning swamp creatures, and my best friend has been avoiding me ever since."
"What were you drinking that night?"
What were you drinking that night? -or- What did you drink at night?
Very different and both right. (Ambiguous without context.)
Wouldn't it be, "in dieser Nacht" as "diese" is more specific...this,that.
diese Nacht is "this night" -- but not usually "that night".
For that, you need die Nacht (with emphasis on die to show that it means "that" rather than "the").
Well, night and evening are a good bit different: the night comes after the evening. Night extends well beyond midnight. You wouldn't call 3:00 neither evening (too late for that) nor morning (too early) because it's the night. :)