Katten løber ud af huset, når døren er åben.

In Danish, the definite article is often attached to the end of the noun instead of being a separate word.

So:

• en kat = a cat

• katten = the cat

• et hus = a house

• huset = the house

• en dør = a door
• døren = the door

That is one of the first big differences English speakers notice. English usually puts the before the noun, but Danish often adds a definite ending to the noun itself.

That depends on the noun’s grammatical gender.

Danish has two genders:

• common gender: usually takes en in the indefinite form and -en in the definite form
• neuter gender: usually takes et in the indefinite form and -et in the definite form

In this sentence:

• en kat → katten
• en dør → døren
• et hus → huset

So kat and dør are common gender nouns, while hus is a neuter noun.

Both løber and er are in the present tense.

• løber = present tense of at løbe
• er = present tense of at være

A very helpful thing for English speakers is that Danish verbs do not change according to person the way English verbs sometimes do.

For example:

• jeg løber = I run
• du løber = you run
• katten løber = the cat runs
• de løber = they run

The verb form stays the same.

Ud af is the natural combination here because it expresses movement from the inside to the outside of something.

• ud = out
• af = of/from
• ud af huset = out of the house

So the phrase works as a unit: løber ud af huset.

You can think of it as matching English run out of the house. Using only ud would feel incomplete if you want to say what the cat is going out of.

Here når means something like when or whenever.

In this sentence, it expresses a general situation or repeated pattern:

• the cat runs out of the house whenever the door is open

That is why når fits well.

Compare:

• når = when/whenever, often for something general, repeated, or expected
• hvis = if, often for a more hypothetical condition

So:

• Katten løber ud af huset, når døren er åben.
  = This happens when/whenever the door is open.

• Katten løber ud af huset, hvis døren er åben.
  = If the door is open, the cat runs out.
  This is possible too, but it sounds a bit more conditional and less like a general rule.

Because når døren er åben is a subordinate clause, and Danish word order changes in subordinate clauses.

In a normal main clause, Danish usually follows the verb-second rule:

• Katten løber ...

But after a subordinating word like når, the clause normally has:

• subject + finite verb

So:

• når døren er åben

not

• når er døren åben

That second version would sound wrong in standard Danish.

The adjective agrees with the noun in gender and number when it is used after er.

Here the noun is døren, from en dør, which is a common gender singular noun. For common gender singular, the adjective usually appears in its basic form:

• døren er åben

Compare:

• vinduet er åbent
  because vinduet is neuter singular

• dørene er åbne
  because dørene is plural

So the form åben is correct because dør is common gender singular.

The comma separates the main clause from the subordinate clause.

Main clause:

• Katten løber ud af huset

Subordinate clause:

• når døren er åben

So the comma marks that division.

One extra detail: in modern Danish, you may also see this sentence written without the comma before når, depending on the comma system being used. So both styles can occur in real Danish writing.

Yes, absolutely.

You can say:

• Når døren er åben, løber katten ud af huset.

That means the same thing.

But notice the word order in the main clause after the fronted subordinate clause:

• Når døren er åben, løber katten ...

The verb løber comes before the subject katten because Danish main clauses still follow the verb-second rule. The opening subordinate clause takes the first position, so the finite verb comes next.

It most naturally describes a habitual or general situation.

Because of når, the sentence usually means something like:

• whenever the door is open, the cat runs out

So it sounds like a general tendency or repeated behavior, not just one single occasion.

If you wanted to talk about one specific event, Danish would often use a different structure or add more context.