Jeg kan ikke komme ind i trappeopgangen, hvis jeg glemmer mit nøglekort.

Because Danish main clauses usually follow the verb-second pattern. The finite verb goes early in the sentence, and ikke normally comes after that finite verb.

So:

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

Here, kan is the finite verb, so ikke comes after it.

A useful comparison:

• Main clause: Jeg kan ikke komme ind
• Subordinate clause: ... fordi jeg ikke kan komme ind

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

Because kan is a modal verb, and modal verbs in Danish are followed by the bare infinitive.

So you say:

• jeg kan komme
• jeg vil komme
• jeg skal komme

not:

• jeg kan at komme

This is similar to English, where we say I can come, not I can to come.

Here komme ind means get in or gain access, not just physically come inside in a neutral sense.

Because the sentence mentions a nøglekort (key card), the meaning is clearly about access:

• jeg kan ikke komme ind = I can't get in

So komme ind is very natural when talking about locked buildings, doors, gates, entry systems, and so on.

Because the two words do slightly different jobs:

• ind shows movement inward
• i introduces the place you are moving into

So ind i is often like English into.

Examples:

• gå ind i huset = go into the house
• komme ind i bilen = get into the car

In your sentence, komme ind i trappeopgangen means get into the stairwell / entrance stair area.

Because the sentence refers to a specific stairwell/entrance area, so Danish uses the definite form.

• en trappeopgang = a stairwell / an entrance stair area
• trappeopgangen = the stairwell / the entrance stair area

Danish usually adds definiteness as an ending on the noun:

• en bil → bilen
• en trappeopgang → trappeopgangen

So the -en at the end means the.

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

It is made from:

• trappe = stairs / staircase
• opgang = entrance, stairwell, building entry section

Together, trappeopgang means something like stairwell, entry stair area, or apartment building entrance/stair section, depending on context.

A useful thing to know: in Danish compounds, the last part usually decides the grammatical gender and the basic meaning.

Hvis means if and introduces a condition.

So:

• hvis jeg glemmer mit nøglekort = if I forget my key card

This presents forgetting the key card as a possibility or condition.

By contrast, når usually means when and often suggests something expected, repeated, or seen as more certain.

Compare:

• Hvis jeg glemmer mit nøglekort, kan jeg ikke komme ind.
  = If I forget my key card, I can't get in.

• Når jeg glemmer mit nøglekort, kan jeg ikke komme ind.
  = When/whenever I forget my key card, I can't get in.

So hvis is the natural choice for a simple conditional sentence like this one.

Because Danish often uses the present tense in conditional clauses about the future, just like English does.

So:

• hvis jeg glemmer mit nøglekort = if I forget my key card

This does not mean only present-time forgetting. It can refer to a future situation.

English works similarly:

• If I forget my key card tomorrow, I can't get in.

So the present tense here is completely normal.

Because nøglekort is a neuter noun in Danish.

Its indefinite form is:

• et nøglekort = a key card

And with a neuter singular noun, the possessive is:

• mit nøglekort = my key card

Compare:

• en nøgle → min nøgle
• et nøglekort → mit nøglekort

So the choice between min, mit, and mine depends on the noun that follows.

Because Danish normally writes compound nouns as one word.

So:

• nøglekort = key card
• lejlighedsdør = apartment door
• adgangskort = access card

This is different from English, where compounds may be written as one word, two words, or hyphenated. In Danish, writing them as one word is very common and usually the correct choice.

Because a possessive like min/mit/mine normally replaces the article.

So Danish says:

• mit nøglekort = my key card

not:

• et mit nøglekort
• det mit nøglekort

This is similar to English, where we say my key card, not the my key card.

Because hvis jeg glemmer mit nøglekort is a subordinate clause.

Many Danish writers place a comma before a subordinate clause like this:

• Jeg kan ikke komme ind i trappeopgangen, hvis jeg glemmer mit nøglekort.

You may also see Danish written without that comma, depending on the punctuation style being used. So the comma here is normal, but you should know that Danish comma rules allow some variation.

Yes, absolutely. That is also correct.

When the hvis-clause comes first, Danish keeps the finite verb in second position in the main clause, so you get inversion:

• Hvis jeg glemmer mit nøglekort, kan jeg ikke komme ind i trappeopgangen.

Notice that it becomes:

• kan jeg

  not:

• jeg kan

This is a very common Danish word-order pattern.