Hendes datter øver violin, mens hendes søn slår på trommer i rummet ved siden af.

The key word is øver, from at øve = to practise.

• Hun øver violin = she is practising violin
• Hun spiller violin = she plays the violin

So øver focuses on rehearsal or practice, not just the general ability or activity. In this sentence, the idea is that the daughter is actively practising.

In Danish, names of musical instruments often appear without an article when talking about learning, playing, or practising them in a general sense.

So:

• øver violin
• spiller klaver
• spiller guitar

This works much like English play piano in some older or informal styles, though standard English usually says play the piano. Danish simply does not need the article here.

Yes, you can, but it has a specific feel.

• slår på trommer literally means hits/beats on drums
• spiller trommer means plays drums

So slår på trommer is more physical and vivid. It can suggest noise, rhythm, or the action of drumming itself. Spiller trommer would be the more neutral way to say someone plays the drums.

In this sentence, slår på trommer makes the scene feel a bit more lively.

Because drums are normally thought of as a set, not a single instrument.

• en tromme = a drum
• trommer = drums

So when someone is playing a drum kit, Danish commonly uses the plural, just like English does in play the drums.

This is a very common Danish question.

Sin/sit/sine is only used when the possessor refers back to the subject of the same clause.

Here, in Hendes datter, the subject is datter. The possessor is some other female person, meaning her daughter. Since the possessor is not the subject, Danish uses hendes, not sin.

Likewise in hendes søn, the possessor is again her, not the subject of that clause.

Compare:

• Anna elsker sin søn = Anna loves her own son
• Anna elsker hendes søn = Anna loves her son (some other woman’s son)

So hendes is correct here.

It is repeated because each noun phrase needs its own determiner:

• hendes datter
• hendes søn

If you removed the second hendes, the sentence would sound incomplete or would need to be restructured.

Danish does not normally do the English-style shortcut of using one possessive for two separate noun phrases in a sentence like this unless the whole phrase is built differently.

Mens means while.

It connects two actions happening at the same time:

• Hendes datter øver violin
• mens hendes søn slår på trommer

So the full idea is: one child is practising violin while the other is drumming.

Mens introduces a subordinate clause, but in this sentence the word order looks fairly straightforward because the subject hendes søn comes right after mens.

It means in the room next door or in the room beside it.

Breakdown:

• i rummet = in the room
• ved siden af = next to / beside

Together, it describes the son as being in the adjacent room.

This is a fixed expression, so ved siden af should be learned as a unit.

Because ved siden af is a multi-word expression meaning next to or beside.

It is not one single word in Danish. You have to keep all the parts:

• ved siden af huset = next to the house
• ved siden af = next door / beside

In your sentence, there is no noun directly after it, because the phrase works adverbially to describe location: in the room next door.

Because mens introduces a subordinate clause, and Danish normally uses a comma to separate clauses.

So:

• Hendes datter øver violin, mens hendes søn slår på trommer ...

This comma is very natural in written Danish and helps show where the second clause begins.

Not especially in this sentence.

The second clause is:

• mens hendes søn slår på trommer i rummet ved siden af

This is a subordinate clause introduced by mens, and the order is:

• conjunction: mens
• subject: hendes søn
• verb: slår

That is completely normal here.

A learner should just note that Danish subordinate clauses often differ from main clauses in adverb placement, but this sentence does not show a tricky case.

Yes. The phrase can mean either beside or next door, depending on context.

For example:

• Hun sidder ved siden af mig = She is sitting beside me
• i rummet ved siden af = in the room next door / in the room next to it

So the context tells you which English translation fits best.