Hvis frimærket mangler, kommer brevet ikke frem, og så må jeg gå til postkassen igen.

Because the sentence starts with a subordinate clause: Hvis frimærket mangler.

After a subordinate clause comes a main clause, and in Danish main clauses the finite verb normally has to be in second position. That causes inversion:

• Hvis frimærket mangler, kommer brevet ikke frem.

Literally, the structure is:

• If the stamp is missing, arrives the letter not...

That sounds wrong in English, but it is normal in Danish.

If the sentence started directly with the subject, you would get normal subject-verb order:

• Brevet kommer ikke frem, hvis frimærket mangler.

So both are correct, but the word order changes depending on what comes first.

At komme frem is a very common Danish expression. Here it means something like:

• to arrive
• to get through
• to reach its destination
• for a letter, to be delivered successfully

So brevet kommer ikke frem means the letter will not make it to where it is supposed to go.

It is not a literal come forward in this context.

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

So:

• Brevet kommer ikke frem
• Jeg må ikke gå
• Han er ikke hjemme

That is why you get:

• kommer brevet ikke frem

The order is:

1. finite verb: kommer
2. subject: brevet
3. negation: ikke

This is a very common Danish pattern.

Here mangler means is missing.

So:

• Hvis frimærket mangler = If the stamp is missing

The verb is at mangle, which often means:

• to be lacking
• to be missing
• to need
• to lack

Examples:

• Jeg mangler penge. = I need / am short of money.
• Der mangler en side i bogen. = A page is missing from the book.
• Frimærket mangler. = The stamp is missing.

So in this sentence, the stamp itself is the thing that is absent.

Because Danish usually puts the definite article at the end of the noun instead of using a separate word like English the.

So:

• et frimærke = a stamp

• frimærket = the stamp

• et brev = a letter

• brevet = the letter

• en postkasse = a mailbox / postbox
• postkassen = the mailbox / postbox

The ending depends on the noun’s gender:

• en-word nouns usually take -en in the definite singular
• et-word nouns usually take -et in the definite singular

So here:

• frimærke is an et-word
• brev is an et-word
• postkasse is an en-word

For the same basic reason as kommer brevet earlier: Danish main clauses follow the verb-second rule.

In og så må jeg gå til postkassen igen, the word så takes the first position in that clause, so the finite verb må must come second:

• og så må jeg gå...

If you removed så, you could say:

• og jeg må gå til postkassen igen

But once så is placed first in that clause, inversion happens:

• og så må jeg gå...

So the pattern is:

• first element: så
• second position: må
• then subject: jeg

Here så means something like:

• then
• in that case
• so

It connects the second idea to the first:

• If the stamp is missing, the letter will not arrive, and then / so I have to go to the mailbox again.

It gives the sentence a natural sense of consequence.

It can correspond to both, but in this sentence have to is often the most natural English translation.

• så må jeg gå til postkassen igen
  = then I have to go to the mailbox again

At måtte often expresses necessity or something unavoidable.

Depending on context, it can also sound like:

• then I’ll have to go to the mailbox again

That often feels especially natural in English here.

Postkasse means mailbox or postbox, depending on context.

In this sentence, it most likely means a public mailbox / postbox where you go to mail a letter.

So:

• gå til postkassen = go to the mailbox / postbox

English speakers sometimes imagine the mailbox at someone’s house, but in this kind of sentence Danish often suggests the box where you post the letter.

The preposition til often means to or toward, and it is very common after gå when you mean going to a place.

So:

• gå til postkassen = go to the mailbox
• gå til stationen = go to the station
• gå til lægen = go to the doctor

In practice, this is just a normal Danish way to express destination.

Igen means again.

So:

• gå til postkassen igen = go to the mailbox again

Putting igen at the end is very natural in Danish. It usually comes late in the clause, especially after the main verb phrase.

Compare:

• Jeg gør det igen. = I do it again.
• Han kommer igen i morgen. = He’s coming again tomorrow.

So the placement here is completely normal.

Not directly. Hvis frimærket mangler focuses on the stamp being absent.

It means:

• if the stamp is missing
• if there is no stamp on it

It does not specifically say who caused that situation.

If you wanted to say if I forgot the stamp, Danish would use a different structure, for example one involving glemme.

So this sentence describes the result of there being no stamp, not the speaker’s action of forgetting it.

The commas help separate the clauses:

• Hvis frimærket mangler,
• kommer brevet ikke frem,
• og så må jeg gå til postkassen igen.

The first comma marks the end of the if-clause.
The second comma separates the next clause from the coordinated clause introduced by og.

Even if punctuation rules are not the main thing you are studying yet, it is useful to notice that the commas reflect the sentence structure very clearly.

Yes. In kommer brevet ikke frem, brevet is still the subject.

Danish word order can move the subject after the verb because of the verb-second rule, but the grammatical role does not change.

So:

• Brevet kommer ikke frem
  and
• Hvis frimærket mangler, kommer brevet ikke frem

both mean that the letter is the thing that does not arrive.

The subject is identified by grammar, not just by being the first word.