Jeg kan ikke komme ind i lejligheden, hvis nøglekortet ikke virker.

In a normal Danish main clause, the finite verb usually comes early, and ikke usually comes after that finite verb.

So:

• Jeg kan ikke komme ind... = correct
• Jeg ikke kan komme ind... = not normal in a main clause

Here:

• Jeg = subject
• kan = finite verb
• ikke = negation
• komme = infinitive

A useful pattern is:

• Subject + finite verb + ikke + infinitive/rest

Examples:

• Jeg kan ikke gå.
• Hun vil ikke spise.

Because hvis nøglekortet ikke virker is a subordinate clause.

In Danish, main clauses and subordinate clauses often place ikke differently:

• Main clause: Jeg kan ikke komme ind
• Subordinate clause: hvis nøglekortet ikke virker

In subordinate clauses, ikke usually comes before the finite verb.

So compare:

• Kortet virker ikke. = main clause
• ... hvis kortet ikke virker. = subordinate clause

That difference is very important in Danish word order.

Komme ind i means get into or come into.

It is built from:

• komme = come / get
• ind = in, inside
• i = in / into

Together, komme ind i lejligheden means get into the apartment.

English often uses a similar idea:

• come in
• get in
• get into

Danish often uses both the direction word and the preposition:

• komme ind i huset = get into the house
• gå ind i rummet = go into the room

So even though ind and i may feel slightly repetitive to an English speaker, this is very natural in Danish.

Because lejligheden means the apartment, while en lejlighed means an apartment.

Danish usually adds the definite article to the end of the noun:

• en lejlighed = an apartment
• lejligheden = the apartment

This is one of the biggest differences from English. Instead of a separate word like the, Danish often uses a suffix:

• bogen = the book
• bilen = the car
• lejligheden = the apartment

In this sentence, a specific apartment is meant, so the definite form is used.

Because Danish very often forms compound nouns by writing them as one word.

So:

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

Then the definite ending is added:

• nøglekortet = the key card

This is completely normal in Danish. English often writes similar things as two words, but Danish prefers one compound word.

More examples:

• togstation = train station
• arbejdsdag = workday
• køkkenbord = kitchen table

Because Danish nouns belong to different grammatical genders, and the definite ending depends on the gender.

Very simply:

• common gender nouns often take -en
• neuter nouns often take -et

So:

• en lejlighed → lejligheden
• et nøglekort → nøglekortet

This also tells you the indefinite article:

• en lejlighed
• et nøglekort

So the endings are not random; they match the noun’s gender.

Here virker means works or functions.

So:

• nøglekortet virker = the key card works
• nøglekortet ikke virker = the key card does not work

The verb virke is very common for machines, devices, systems, cards, buttons, apps, and so on.

Examples:

• Min telefon virker ikke. = My phone isn’t working.
• Låsen virker. = The lock works.
• Kortet virker ikke. = The card doesn’t work.

Hvis means if.

It introduces a condition:

• hvis nøglekortet ikke virker = if the key card doesn’t work

So the sentence has:

• a main clause: Jeg kan ikke komme ind i lejligheden
• a condition clause: hvis nøglekortet ikke virker

This is very similar to English if.

Yes. You can also say:

Hvis nøglekortet ikke virker, kan jeg ikke komme ind i lejligheden.

This is very natural.

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

• Jeg kan ikke komme ind i lejligheden, hvis nøglekortet ikke virker.
• Hvis nøglekortet ikke virker, kan jeg ikke komme ind i lejligheden.

After the initial hvis-clause, Danish uses verb-second order in the main clause, so you get:

• kan jeg not
• jeg kan

That is a very common Danish pattern.

Because after a modal verb like kan, Danish uses the bare infinitive.

So:

• kan komme = can come / can get
• not kan at komme

This is similar to English:

• I can go
• not I can to go

Other common modal verbs work the same way:

• vil gå = want to go / will go
• skal arbejde = must / will work
• må vente = may / must wait

So kan komme is exactly what you should expect.

No. They are quite different.

• kan ikke = cannot / am not able to
• må ikke = must not / am not allowed to

So in this sentence:

• Jeg kan ikke komme ind i lejligheden = I can’t get into the apartment

This sounds like lack of ability or possibility, probably because the key card does not work.

If you said:

• Jeg må ikke komme ind i lejligheden

that would mean:

• I am not allowed to enter the apartment

So the meaning changes a lot.

It is very natural, but there are other possible ways depending on nuance.

For example:

• Jeg kan ikke komme ind i lejligheden = I can’t get into the apartment
• Jeg kan ikke komme ind = I can’t get in
• Jeg kan ikke låse mig ind i lejligheden = I can’t unlock my way into the apartment / I can’t get into the apartment by unlocking it

The original sentence is a very normal, everyday way to say it.

A few very common mistakes would be:

1. Putting ikke in the wrong place

   • wrong: Jeg ikke kan komme ind...
   • correct: Jeg kan ikke komme ind...

2. Using main-clause word order after hvis

   • wrong: hvis nøglekortet virker ikke
   • correct: hvis nøglekortet ikke virker

3. Forgetting the definite endings

   • wrong: i lejlighed
   • correct: i lejligheden
   • wrong: nøglekort virker ikke
   • correct: nøglekortet virker ikke

4. Writing compounds as separate words

   • wrong: nøgle kortet
   • correct: nøglekortet

These are all very normal learner mistakes, so they are worth watching out for.