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.

En nabo means a neighbor, so the person is not identified as a specific, already known neighbor.

If it were naboen, it would mean the neighbor.

Also, en tells you that nabo is a common-gender noun. A neuter noun would use et instead.

Because in Danish, a possessed noun normally does not take en, et, or the definite ending.

So you get:

• min bil = my car
• hans hus = his house
• hvis postkasse = whose mailbox

That is why it is postkasse, not en postkasse or postkassen.

Danish often uses stå for things that are upright or positioned somewhere in a fixed way.

So a mailbox can stå ved døren, just as a bottle can stå på bordet.

English often just uses is in similar sentences, but Danish is more likely to describe the physical position more specifically.

Ved means by, near, or next to.

So ved den røde dør means the mailbox is located by the red door.

That is different from på den røde dør, which would mean on the red door, physically attached to its surface.

Because the adjective changes form in a definite noun phrase.

Compare:

• en rød dør = a red door
• den røde dør = the red door

When Danish uses den, det, or de before an adjective and noun, the adjective usually takes -e. That is why it is røde.

Taler is present tense.

The infinitive is at tale = to speak / to talk.

So:

• at tale = infinitive
• taler = present tense

In context, Hun taler med en nabo can mean either She speaks with a neighbor or She is talking with a neighbor, depending on the situation.

Tale med usually suggests a two-way conversation, so it matches talking with someone.

Tale til is more like speaking to someone, and it can sound more one-directional.

So in this sentence, Hun taler med en nabo naturally suggests that she and the neighbor are having a conversation.

Because this is a subordinate clause, not a main clause.

In Danish main clauses, the verb often comes in second position. But in subordinate clauses, the subject usually comes before the verb.

So in:

hvis postkasse står ved den røde dør

the order is:

• postkasse = subject
• står = verb

That is the normal word order for this kind of clause.