Det virker mærkeligt, at døren er åben, når ingen er hjemme.

Here Det works like English it in expressions such as It seems strange. It does not refer to one specific thing in the same way a normal noun does; instead, it points to the whole situation described afterward.

So:

Det virker mærkeligt = It seems strange

And then the at-clause explains what is strange:

at døren er åben = that the door is open

Both can often be translated as seems strange / is strange, but there is a nuance:

• Det er mærkeligt = It is strange
  This sounds more direct and definite.
• Det virker mærkeligt = It seems strange
  This sounds a little softer, more like the speaker’s impression.

So virker is often used when something appears a certain way rather than being stated as a hard fact.

The adjective mærkelig changes form here because it is used with det.

Compare:

• en mærkelig dør = a strange door
• døren er mærkelig = the door is strange
• det er mærkeligt = it is strange

With det or with a whole situation/clause as the subject, Danish normally uses the -t form of the adjective.

So in Det virker mærkeligt, the correct form is mærkeligt.

Because dør is a common gender noun in Danish.

In predicative position, adjectives agree with the noun in gender and number:

• common gender singular: åben
• neuter singular: åbent
• plural: åbne

So:

• døren er åben = correct
• huset er åbent = correct
• dørene er åbne = correct

Even though døren is definite, the adjective after er does not take a special definite form here.

Døren means the door. The sentence is talking about a specific door, not just any door.

Danish usually adds the definite ending directly to the noun:

• en dør = a door
• døren = the door

This is very common in Danish. Instead of a separate word like English the, Danish often marks definiteness with an ending.

At introduces a subordinate clause, just like English that:

at døren er åben = that the door is open

In English, that is often optional:

• It seems strange that the door is open
• It seems strange the door is open

In Danish, at is much more natural here and is usually included.

Når usually means when and is used for something seen as a real situation, a repeated situation, or a general condition.

Hvis means if and is more clearly conditional or hypothetical.

So:

• når ingen er hjemme = when no one is home / whenever no one is home
• hvis ingen er hjemme = if no one is home

In your sentence, når suggests a real-world circumstance: the door being open is strange in the situation where nobody is at home.

Ingen means no one / nobody / none. It already contains the negative meaning.

So:

• ingen er hjemme = no one is home

You do not add ikke here, because ingen is already negative.

Compare:

• Nogen er hjemme = someone is home / anyone is home depending on context
• Ingen er hjemme = no one is home

A useful rule: ingen is the negative form, while nogen is the non-negative form.

Because når introduces a subordinate clause, and subordinate clauses in Danish normally keep the subject before the finite verb:

• når ingen er hjemme

This is different from main-clause word order, where Danish often follows the verb-second rule.

For example:

• Ingen er hjemme. = main clause
• Når ingen er hjemme, virker det mærkeligt... = subordinate clause first, then main clause

Inside the når-clause itself, the order is normal subordinate-clause order: subject + verb.

Hjemme means at home and describes being in that location.

• Han er hjemme = He is at home

Hjem usually refers to motion toward home or the idea of home:

• Han går hjem = He is going home

So in your sentence:

• ingen er hjemme = no one is at home

You need hjemme because this is about location, not movement.

This sentence is useful because it contains all three special Danish vowels:

• æ in mærkeligt
• ø in døren
• å in åben

Very roughly:

• æ sounds somewhat like the vowel in English cat, but usually a bit more open or fronted.
• ø has no exact English equivalent. It is somewhat similar to the vowel in French deux or German schön if you know those.
• å sounds roughly like the vowel in English more or saw in many accents, but not exactly.

Also note that Danish pronunciation is often less straightforward than spelling suggests, so it helps to hear native audio. For example, døren may sound more like DUR-en with a very Danish vowel quality rather than a clear English-style sound.

Yes. Danish allows you to move the når-clause to the front:

• Når ingen er hjemme, virker det mærkeligt, at døren er åben.

This means the same thing, but the emphasis changes slightly. Starting with Det virker mærkeligt focuses first on the speaker’s reaction. Starting with Når ingen er hjemme sets up the situation first.

Both are natural Danish.