Det lille rum er mindre end stuen.

Because rum is a neuter noun in Danish.

Danish nouns have two genders:

• common gender → usually takes en
• neuter gender → usually takes et

So:

• et rum = a room
• det lille rum = the small room

Here, det matches the neuter noun rum. It is not the pronoun it in this sentence.

Because Danish normally uses a special pattern for definite nouns with an adjective.

Compare:

• rummet = the room
• det lille rum = the small room

When there is no adjective, Danish often puts definiteness on the end of the noun:

• rummet

When there is an adjective before the noun, Danish usually adds a separate definite article in front:

• det lille rum

So det lille rummet is generally not correct in standard Danish.

Because lille is the form used here for small/little.

This adjective is a bit special. In this kind of phrase, Danish uses:

• det lille rum = the small room

You should learn these common forms:

• lille = small/little
• mindre = smaller
• mindst = smallest

So even though the basic meaning is small, the form you see here is lille.

Because mindre is the irregular comparative form of lille.

In English, this is similar to how good becomes better, not gooder.

So:

• lille = small
• mindre = smaller
• mindst = smallest

You simply have to memorize this pattern.

End means than in comparisons.

So:

• mindre end = smaller than

This is the normal word used after a comparative in Danish:

• større end = bigger than
• ældre end = older than
• mindre end = smaller than

Because stuen is the definite form of stue.

So:

• en stue = a living room / a sitting room
• stuen = the living room

The sentence is comparing the small room to a specific living room, so Danish uses the definite form stuen.

Because Danish often puts the at the end of the noun.

So instead of using a separate word like English the room, Danish often says:

• rummet = the room
• stuen = the living room

That ending is what makes the noun definite.

This is different from det lille rum, where the adjective changes the pattern and requires a separate article in front.

The sentence has a very normal Danish main-clause word order:

• Det lille rum = subject
• er = verb
• mindre end stuen = complement

So the structure is basically:

• [subject] + [verb] + [rest]

This matches English quite closely:

• The small room is smaller than the living room.

No. In this sentence, det is part of the noun phrase det lille rum.

It works as the definite article for a neuter singular noun with an adjective:

• det lille rum = the small room

If det meant it, the structure would be different.

Then you would usually use den instead of det.

For example:

• den lille stol = the small chair

So the pattern is:

• det
  • adjective + neuter noun
• den
  • adjective + common-gender noun

In your sentence, rum is neuter, so it must be det lille rum.