Døren åbnes med nøglekortet.

Because døren is the definite singular form: the door.

In Danish, the definite article is usually attached to the end of the noun:

• en dør = a door
• døren = the door

Here, dør is a common-gender noun, so the definite ending is -en.

For the same reason: it is the definite singular form, meaning the key card.

• et nøglekort = a key card
• nøglekortet = the key card

The ending is -et because nøglekort is a neuter noun.

Because Danish, like German, very often forms compound nouns as a single word.

So:

• nøgle = key
• kort = card
• nøglekort = key card

This is completely normal in Danish. The last part of the compound usually determines the grammatical gender, so because kort is neuter (et kort), we get et nøglekort.

Åbnes is the passive form of åbne.

• at åbne = to open
• åbner = opens / is opening
• åbnes = is opened

That -s is a very common way to make the passive in Danish. So the sentence is using the present passive.

A natural English equivalent is:

• The door is opened with the key card

Depending on context, English might phrase it more naturally as:

• The door opens with the key card
• Use the key card to open the door

Yes. Very common.

Danish often uses the -s passive in:

• instructions
• technical descriptions
• formal language
• signs and notices

So Døren åbnes med nøglekortet sounds natural if you are describing the normal method for opening the door.

Yes. That is also correct.

There are two common passive patterns in Danish:

• -s passive: Døren åbnes
• blive-passive: Døren bliver åbnet

Very roughly:

• -s passive often sounds more general, impersonal, formal, or instructional
• bliver + past participle can sound more like a specific event or process

In a sentence like this, the -s passive is especially natural.

Because med shows the instrument or means: with / by using.

Here, the key card is the tool used to open the door, so:

• med nøglekortet = with the key card

By contrast, af is usually used for the agent in a passive sentence, especially a person or force doing the action:

• Døren åbnes af vagten = The door is opened by the guard

So:

• med = tool/method
• af = doer/agent

Because in Danish, the is usually added to the end of the noun instead of appearing as a separate word.

So:

• døren already means the door
• nøglekortet already means the key card

A separate definite word like den or det is usually used when there is an adjective:

• døren = the door
• den store dør = the big door

Yes, it is in the present tense, but that does not always mean something is happening right now.

In Danish, the present tense is often used for:

• general facts
• instructions
• habitual actions
• how something works

So here it most likely means something like:

• The door is opened with the key card
• The door opens using the key card
• To open the door, use the key card

It describes the normal method, not necessarily an action happening at this exact moment.

The sentence follows normal Danish main-clause word order:

• Døren = subject
• åbnes = verb
• med nøglekortet = prepositional phrase

So the pattern is:

• Subject + Verb + other information

Danish main clauses usually put the finite verb in the second position.

If you move something else to the front, the verb still stays second:

• Normalt åbnes døren med nøglekortet. = Normally, the door is opened with the key card.

Notice that after fronting Normalt, the verb åbnes still comes second.

Yes, grammatically it would be correct, but the meaning changes.

• Døren åbnes med nøglekortet = The door is opened with the key card
• En dør åbnes med et nøglekort = A door is opened with a key card

The version with definite forms sounds more like a specific known door and a specific known key card, or the standard door and card in a particular setting. That is why it is more natural in many real contexts, such as instructions in a building or hotel.

A rough guide is:

DØ-ren AWB-nes med NØG-le-kor-tet

A few useful notes:

• ø is a rounded vowel that English does not really have
• å sounds somewhat like the vowel in more for many learners, though not exactly
• g in nøgle is not pronounced like a strong English g
• the definite endings -en and -et are often pronounced quite lightly in natural speech

If you want to sound natural, the hardest parts are usually døren, åbnes, and nøglekortet.