Breakdown of Hun taler med en nabo, hvis postkasse står ved den røde dør.
Questions & Answers about Hun taler med en nabo, hvis postkasse står ved den røde dør.
Here, hvis means whose. It introduces a relative clause and shows possession:
en nabo, hvis postkasse står ved den røde dør
= a neighbor whose mailbox is by the red door
Danish hvis can also mean if, but not in this sentence. The grammar here clearly shows the whose meaning.
It refers back to en nabo.
So the sentence is talking about a neighbor, and then adds more information about that neighbor: the neighbor is the one whose mailbox stands by the red door.
Because hvis postkasse står ved den røde dør is a relative clause giving extra information about en nabo.
This sentence uses a start comma, which is very common and fully acceptable in Danish. So the comma marks the beginning of the subordinate clause.