Postnummeret på pakken var forkert, så den kom først frem i dag.

Because postnummeret is the definite form of et postnummer: the postal code / the postcode.

In Danish, definiteness is often added as an ending:

• et postnummer = a postal code
• postnummeret = the postal code

So Postnummeret på pakken means the postal code on the package.

Yes. På pakken literally means on the package.

That is very natural in Danish when talking about something written or attached on the outside of an object:

• adressen på brevet = the address on the letter
• mærket på flasken = the label on the bottle
• postnummeret på pakken = the postal code on the package

English might sometimes say for the package or on the package, but Danish clearly uses på here.

Because pakken is the definite form of en pakke:

• en pakke = a package
• pakken = the package

So på pakken means on the package.

This is the same definite-ending pattern as in many Danish nouns:

• en bil → bilen
• en bog → bogen
• en pakke → pakken

It is forkert because it agrees with postnummeret, and postnummer is a neuter noun: et postnummer.

When an adjective is used predicatively after verbs like være, Danish often uses:

• common gender singular: base form
• neuter singular: often -t
• plural/definite: often -e

So:

• Nummeret er forkert = the number is wrong
• Adressen er forkert = the address is wrong
• Postnummeret er forkert = the postcode is wrong

Here, the -t matches the neuter noun et postnummer.

Because the sentence is describing a past situation.

• er = present tense, is
• var = past tense, was

So:

• Postnummeret ... er forkert = the postcode is wrong
• Postnummeret ... var forkert = the postcode was wrong

The whole sentence is telling what happened before the package finally arrived today, so past tense is natural.

Here så means so, in the sense of therefore / as a result.

It connects two main clauses:

• Postnummeret på pakken var forkert
• så den kom først frem i dag

So the logic is: The postcode on the package was wrong, so it only arrived today.

This så is a coordinating conjunction, similar to English so.

Because så here is a coordinating conjunction joining two main clauses, and that does not trigger inversion.

So the normal subject-verb order stays:

• den kom

Compare:

• Postnummeret var forkert, så den kom først frem i dag. = The postcode was wrong, so it only arrived today.

But if så means something more like then, used as an adverb at the start of a clause, you often do get inversion:

• Så kom den frem. = Then it arrived.

So in your sentence, så = so / therefore, not then.

Den refers to pakken.

Since pakke is a common-gender noun (en pakke), the pronoun is den:

• en pakke → den
• et brev → det

So:

• Pakken kom frem → Den kom frem

That is why the sentence says så den kom først frem i dag.

Komme frem is a common expression meaning to arrive, to get through, or to reach its destination, depending on context.

With mail or packages, kom frem means something like:

• arrived
• made it through
• reached the recipient

So:

• Pakken kom frem = The package arrived
• Brevet kom aldrig frem = The letter never arrived

The word frem adds the idea of reaching the end point.

Because kom frem is the natural idiomatic expression for mail, parcels, messages, and similar things reaching their destination.

If you say just kom, that usually only means came, which is more general and can sound incomplete here.

So:

• Den kom i dag = it came today
• Den kom frem i dag = it arrived / got through today

With packages and letters, kom frem is the more precise and natural choice.

Here først means only in the sense of not until.

So:

• den kom først frem i dag

means:

• it only arrived today
• it didn’t arrive until today

This is a very common use of først in Danish.

For example:

• Jeg er først hjemme klokken otte. = I won’t be home until eight.
• Hun fandt det først senere. = She only found it later.

Because først is modifying the whole idea of the package’s arrival time: it only arrived today.

The structure is:

• den kom først frem i dag

This is natural Danish word order. Først comes before the particle/adverb frem and before the time expression i dag.

It is essentially saying:

• it came only-arrived today

which English renders more naturally as:

• it only arrived today
• it didn’t arrive until today

I dag means today.

Danish uses i dag as a fixed time expression:

• i dag = today
• i morgen = tomorrow
• i går = yesterday

There is no article because it functions like an adverbial time expression, not like a noun phrase such as the day.

So:

• Den kom frem i dag = It arrived today

Yes, ankom is possible, but kom frem is more everyday and idiomatic for letters and packages.

Compare:

• Pakken kom frem i dag. = natural, conversational
• Pakken ankom i dag. = also correct, a bit more formal or neutral

For ordinary spoken Danish, kom frem often sounds more natural in this context.

Yes, in meaning it corresponds to postcode / postal code / ZIP code, depending on the variety of English.

• In British English, postcode
• In general/international English, postal code
• In American English, usually ZIP code

So postnummeret here is the number used for mail delivery.

Yes:

• Postnummeret = the postal code
• på pakken = on the package
• var forkert = was wrong
• så = so
• den = it
• kom frem = arrived / got through
• først i dag = only today / not until today

So the structure is:

Postnummeret på pakken
subject phrase

var forkert
predicate

så
link: so

den kom først frem i dag
result clause: it only arrived today