Learn Hardcore Danish: Værelset er stille om aftenen.

stille focuses on sound level – little or no noise, quiet, silent.
rolig focuses on mood/atmosphere or behavior – calm, relaxed, not hectic.

In your sentence:

Værelset er stille om aftenen.
Emphasizes that it is quiet, not noisy.

You could say:

Værelset er roligt om aftenen.

That would suggest the room has a calm, peaceful atmosphere, which may or may not be exactly what you want to say. Stille is the more direct translation of quiet for sound.

Why doesn’t stille change form? Shouldn’t it agree with værelset somehow?

Many Danish adjectives change form for gender/number, but stille is one of the adjectives that are invariable: it has the same form for singular, plural, common, and neuter.

Compare:

huset er stort (the house is big – stort is the neuter form of stor)
værelset er stille (the room is quiet – stille doesn’t change)

So stille looks the same in all positions:

et stille værelse = a quiet room
værelset er stille = the room is quiet
stille værelser = quiet rooms

What exactly does værelse mean? Is it only a bedroom?

Værelse is a general word for room in a building. Context tells you which kind:

et værelse – a room (often a bedroom, but not necessarily)
et hotelværelse – a hotel room
et børneværelse – a children’s room / kid’s bedroom

If you want to be specific:

et soveværelse = a bedroom
en stue = a living room

In your sentence, værelset can be understood as the room you’re talking about (e.g. your bedroom, a hotel room, a rented room).

Why is aftenen with -en? Is that also a definite form?

Yes. Aften (evening) is a common-gender noun:

en aften = an evening
aftenen = the evening

In the phrase om aftenen, it is grammatically definite (literally “about/on the evening”), but in English we translate it more naturally as in the evening / in the evenings.

This is a set pattern:

om morgenen – in the morning(s)
om dagen – in the daytime
om natten – at night
om aftenen – in the evening(s)

Could I change the word order to Om aftenen er værelset stille?

Yes, that is perfectly correct and very natural.

Danish has a V2 rule: the finite verb (here er) must be in second position in a main clause. Both versions respect that:

Værelset (1) er (2) stille om aftenen
Om aftenen (1) er (2) værelset stille

The meaning is the same; putting om aftenen first gives it a bit more emphasis (“As for the evenings, then the room is quiet”).

I learned that om means about. Why does it mean in here?

Om is a very flexible preposition in Danish. Some common uses:

tale om noget – talk about something
en bog om krigen – a book about the war
om fem minutter – in five minutes
om aftenen – in the evening(s)

In time expressions, om often means in / at / during, especially for:

parts of the day (om aftenen, om morgenen)
future time spans (om en uge = in a week)

So the meaning depends on context.

How would I say “The rooms are quiet in the evening” (plural)?

You need the plural forms of værelse and the plural verb form (which happens to look the same as the singular):

et værelse – a room
værelset – the room
værelser – rooms
værelserne – the rooms

So:

Værelserne er stille om aftenen.
→ The rooms are quiet in the evening(s).

How do you pronounce værelset and aftenen?

Approximate pronunciations (in simple English terms):

værelset ≈ VAIR-el-set
- væ like “vare” in English “various”, but with a more open æ sound
- the -el- is quite weak and can almost blend with væ-
- final -t is typically soft and may be barely audible in fast speech
aftenen ≈ AF-ten-un
- af like “af” in “after” (short a)
- te like “ten” but very short
- -nen often sounds like a light -nən or -n

Danish reduces and softens a lot of consonants, so the spoken forms are often shorter and smoother than the written ones.