Han må have glemt sit nøglekort i vaskeriet, for han kan ikke komme ind i trappeopgangen.

It means must have forgotten.

In this sentence, må does not mean permission, like may in English. It expresses a logical conclusion:

• Han må have glemt ... = He must have forgotten ...

The structure is:

• må = must
• have = have
• glemt = forgotten

So Danish uses må + have + past participle to make an educated guess about the past.

Because nøglekort is a neuter noun: et nøglekort.

The reflexive possessive pronouns are:

• sin for common gender singular nouns
• sit for neuter singular nouns
• sine for plural nouns

So:

• sit nøglekort = his own key card

It is also reflexive because the possessor is the subject of the clause:

• Han må have glemt sit nøglekort

That means the key card belongs to himself.

If you used hans nøglekort, it would usually mean his key card in a non-reflexive sense, often suggesting someone else’s, or at least not using the normal reflexive form. In this sentence, sit is the natural choice.

Because after må have, Danish uses the past participle.

The forms are:

• at glemme = to forget
• glemte = forgot
• glemt = forgotten

So:

• Han må have glemt ... = He must have forgotten ...

This is the same pattern as English:

• must have forgotten not
• must have forgot and not
• must forgot

It is a compound noun, which is extremely common in Danish.

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

Danish often joins nouns into one word where English might use two words or a noun + noun phrase.

The last part usually determines the grammatical gender, and since kort is neuter (et kort), we get:

• et nøglekort

That is also why the sentence uses sit.

i vaskeriet means in the laundry room or in the laundry area, depending on context.

The base noun is:

• et vaskeri = a laundry / laundromat / laundry room

The definite form is:

• vaskeriet = the laundry room / the laundry

So:

• i vaskeriet = in the laundry room

Danish usually adds the definite article to the end of the noun instead of using a separate word like the.

This sentence probably refers to a specific laundry room that both speaker and listener know about, such as the shared laundry room in an apartment building.

Yes. In this sentence, for means because or for, introducing an explanation:

• Han må have glemt sit nøglekort i vaskeriet, for han kan ikke komme ind ...
• He must have forgotten his key card in the laundry room, because he can’t get in ...

This for is a conjunction, not the English preposition for.

It is often used to give a reason or explanation, especially in a slightly more formal or written style.

You could often also use fordi in similar contexts, but for has its own feel and grammar.

Because for is a coordinating conjunction, so the clause after it keeps main-clause word order.

That is why you get:

• for han kan ikke komme ind ...

with the finite verb kan in normal main-clause position.

A learner often expects subordinate-clause word order after any word meaning because, but for does not work like that. It behaves more like English for or and, connecting two main clauses.

So the structure is:

• main clause: Han må have glemt sit nøglekort i vaskeriet
• main clause after for: han kan ikke komme ind i trappeopgangen

It means cannot get into or cannot get inside.

Breakdown:

• kan = can
• ikke = not
• komme ind = come in / get in
• i = into / in

So:

• kan ikke komme ind i trappeopgangen = cannot get into the stairwell / entrance area

The combination komme ind i is very natural when talking about gaining access to a place.

trappeopgangen means something like:

• the stairwell
• the apartment entrance
• the building’s stairway/entry area

The base noun is:

• en trappeopgang

and the definite form is:

• trappeopgangen = the stairwell / the entrance stairway

In many Danish apartment-building contexts, trappeopgang refers to the shared interior entrance/stair area of the building.

So the sentence means he cannot get into that part of the building, presumably because he needs the key card to access it.