Jeg beholder boardingkortet, i tilfælde af at flynummeret ændres.

Beholde means to keep / retain, while holde usually means to hold or to keep in other senses, depending on context.

So in this sentence:

• Jeg beholder boardingkortet = I keep the boarding pass
• This implies I do not throw it away or I continue to keep it

If you used holder, it would sound like I am holding the boarding pass physically, which is a different idea.

Because Danish often marks the definite form by adding an ending to the noun.

• et boardingkort = a boarding pass
• boardingkortet = the boarding pass

This is very different from English, where you usually add a separate word like the.

Since boardingkort is a neuter noun (et noun), its definite singular ending is -et.

So:

• et boardingkort → boardingkortet

For the same reason: flynummeret is the definite singular form.

• et flynummer = a flight number
• flynummeret = the flight number

In this sentence, Danish uses the definite form because it refers to a specific flight number relevant to the situation — the one on your trip / on the boarding pass.

English sometimes does this too:

• in case the flight number changes

So the Danish definite form is very natural here.

I tilfælde af at means in case.

So:

• i tilfælde af at flynummeret ændres = in case the flight number changes

It is similar to hvis (if), but not exactly the same in tone.

Difference:

• hvis = if
• i tilfælde af at = in case

I tilfælde af at often suggests precaution: you do something now because something might happen later.

That fits this sentence very well:

• Jeg beholder boardingkortet, i tilfælde af at flynummeret ændres.
• I keep the boarding pass as a precaution, in case the flight number changes.

A version with hvis would also be understandable, but it would not sound quite as specifically like a precaution.

Because i tilfælde af by itself is followed by a noun phrase, while i tilfælde af at is used before a whole clause.

Compare:

• i tilfælde af regn = in case of rain
• i tilfælde af at det regner = in case it rains

So in your sentence, what follows is a clause:

• flynummeret ændres

Because it is a clause, Danish uses:

• i tilfælde af at ...

This is a fixed and very common structure.

Yes. Ændres is a passive form.

Active:

• Nogen ændrer flynummeret = Someone changes the flight number

Passive:

• Flynummeret ændres = The flight number is changed / changes

In this sentence, Danish uses the passive because the important thing is the flight number being changed, not who changes it.

This is very common in Danish.

Also, in English we often translate it more naturally as:

• if the flight number changes

even though the Danish form is grammatically passive.

Because Danish often uses the present tense for future meaning when the context already makes the time clear.

Here, the phrase i tilfælde af at already shows that this is about a possible future situation.

So:

• Jeg beholder boardingkortet = literally present, but naturally I’m keeping / I keep
• flynummeret ændres = literally present passive, but naturally the flight number changes / is changed

English does something similar:

• I’ll keep the boarding pass in case the flight number changes

Notice that English also often uses changes, not will change, after in case.

So this tense choice is very natural in both languages.

Because Danish punctuation often places a comma before a subordinate clause.

Here the second part:

• i tilfælde af at flynummeret ændres

contains the subordinate clause at flynummeret ændres.

In standard Danish, it is normal to separate this from the main clause with a comma:

• Jeg beholder boardingkortet, i tilfælde af at flynummeret ændres.

Comma rules in Danish are not always identical to English comma rules, so this is something learners often notice.

Yes — or at least it is a well-established word used in Danish, even though boarding comes from English.

Danish often includes loanwords, especially in travel, technology, and business vocabulary.

So:

• boardingkort is perfectly understandable and common

You may also hear airport-related English-based vocabulary in Danish quite often. That is normal and does not make the sentence less Danish grammatically.

Unfortunately, there is no fully reliable rule that lets you guess every noun gender in Danish. You often have to learn it together with the noun.

So it is best to memorize them as:

• et boardingkort
• et flynummer

Once you know they are et nouns, the definite form becomes predictable:

• boardingkortet
• flynummeret

A good habit is to learn Danish nouns with their article:

• not just boardingkort
• but et boardingkort

That will help you form the definite version correctly later.

Yes. A simpler version might be:

• Jeg beholder boardingkortet, hvis flynummeret ændres.

This means roughly the same thing, but it is a bit less specifically precautionary than i tilfælde af at.

The original sentence sounds slightly more careful and natural in the sense of:

• I’ll keep it just in case

So both are possible, but the original phrasing is a very good choice.