Jeg sover dybt, når rummet er stille.

Because dybt is being used as an adverb, not as an adjective.

• dyb = deep
• dybt = deeply

In Jeg sover dybt, the word dybt describes how someone sleeps, so English uses deeply and Danish uses the adverb form dybt.

Compare:

• et dybt hul = a deep hole
• jeg sover dybt = I sleep deeply

So here, dybt is the correct form.

Because rummet means the room, while rum means a room or just room in a general sense.

Danish often adds the definite article to the end of the noun:

• et rum = a room
• rummet = the room

Since the sentence refers to a specific room, Danish uses rummet.

Because rum is a neuter noun in Danish.

Danish nouns have two grammatical genders:

• en-words (common gender)
• et-words (neuter)

rum is an et-word, so:

• et rum = a room
• rummet = the room

Unfortunately, noun gender usually has to be learned with each noun.

Because stille is the correct adjective form here.

In Danish, many adjectives take -t in the neuter singular, but some adjectives already ending in an unstressed -e do not add another -t. stille is one of those.

So you get:

• et stille rum = a quiet room
• rummet er stille = the room is quiet

Even though rum is neuter, the form stays stille, not stilt.

Because er is the verb to be, and stille is an adjective.

So the structure is:

• rummet = the room
• er = is
• stille = quiet / still

Together: rummet er stille = the room is quiet

Danish often uses to be + adjective just like English.

Når is used for when in a general, repeated, or habitual sense.

This sentence describes something that usually happens:

• I sleep deeply when the room is quiet.

That is not one single past event. It is a general condition or repeated situation, so når is the natural choice.

A very useful contrast is:

• når = when, whenever, in general / in the future
• da = when, at that time, for one specific event in the past

So here når is correct because the meaning is habitual.

Because når introduces a subordinate clause, and Danish word order changes in subordinate clauses.

Main clause:

• Jeg sover dybt
  Subject + verb + other information

Subordinate clause:

• når rummet er stille
  når
  • subject + verb + adjective

So after når, the subject rummet comes before the verb er.

This is normal Danish subordinate-clause word order.

Because Danish normally puts a comma before a subordinate clause.

Here:

• Jeg sover dybt, når rummet er stille.

The part starting with når is a subordinate clause, so the comma is standard Danish punctuation.

English punctuation is often more flexible, but in Danish this comma is much more regular.

Because Danish often uses the present tense for general truths, habits, and regular situations.

So:

• Jeg sover dybt = I sleep deeply / I am sleeping deeply
• når rummet er stille = when the room is quiet

In this sentence, the present tense expresses a usual pattern, not necessarily something happening only right now.

In this sentence, stille is best understood as quiet.

Danish stille can overlap with English quiet, still, or calm, depending on context. But with rummet it usually means that the room is quiet, with little or no noise.

So in rummet er stille, the most natural English meaning is the room is quiet.

A rough pronunciation guide is:

• Jeg ≈ yai
• sover ≈ SOH-vor
• dybt ≈ something like duupt with a soft Danish vowel
• når ≈ nor
• rummet ≈ ROOM-et
• er ≈ a very light air / er
• stille ≈ STIL-uh

A more natural rhythm is:

jeg SO-ver DYBT, når RUM-met er STIL-le

The biggest thing for English speakers is that Danish pronunciation is often much softer and less clearly pronounced than spelling suggests.

Yes. Danish can also put the når-clause first:

• Når rummet er stille, sover jeg dybt.

That means the same thing.

Notice that when the subordinate clause comes first, the main clause changes word order:

• Når rummet er stille, sover jeg dybt.

Not:

• Når rummet er stille, jeg sover dybt.

This happens because Danish main clauses normally follow the verb-second pattern.