Postbuddet ringer på, når pakken er for stor til postkassen.

Postbuddet is the definite form of postbud, so it means the mail carrier / the postman / the postal worker.

In Danish, definiteness is often added as an ending:

• et postbud = a mail carrier
• postbuddet = the mail carrier

So the sentence is talking about the mail carrier in a general, definite sense.

At ringe på means to ring the doorbell or more generally to ring at someone’s door.

So:

• Postbuddet ringer på = The mail carrier rings the doorbell / rings at the door

The word på is part of the expression here. You cannot usually translate it word-for-word as English on. It is just part of the Danish phrase ringe på.

You may also hear:

• Han ringer på døren = He rings at the door / rings the doorbell

In Danish, a comma is normally written before a subordinate clause, and når introduces one here.

So the sentence is divided like this:

• Postbuddet ringer på,
• når pakken er for stor til postkassen.

This is standard Danish punctuation. English is less strict about this kind of comma, so it may feel unusual to an English speaker.

Når is used for something that happens when or whenever a situation occurs, especially something seen as regular or expected.

Here the sentence describes a general rule:

• The mail carrier rings the doorbell when the package is too big for the mailbox.

So når fits well.

By contrast, hvis means if and often introduces a more open condition:

• Hvis pakken er for stor, ringer postbuddet på.
  = If the package is too big, the mail carrier rings the doorbell.

In many real contexts, når and hvis can both be possible, but når often sounds more like this is what normally happens.

Because når pakken er for stor til postkassen is a subordinate clause, and Danish word order changes in subordinate clauses.

In a normal main clause, Danish often has verb-second order:

• Pakken er stor.

But in a subordinate clause introduced by words like når, fordi, at, hvis, the subject usually comes before the verb:

• når pakken er stor
• fordi pakken er stor

So når pakken er ... is the expected Danish structure.

For stor til means too big for.

So:

• Pakken er for stor til postkassen
  = The package is too big for the mailbox

Important distinction:

• stor = big
• meget stor = very big
• for stor = too big

So for here does not mean English for. It means too in the sense of excessively.

Postkassen is the definite form of postkasse, meaning the mailbox.

• en postkasse = a mailbox
• postkassen = the mailbox

So:

• til postkassen = for the mailbox / for the letterbox

The sentence refers to the mailbox, not just any mailbox.

Postbud is perfectly understandable and still used, but some learners also come across words like:

• postmand = postman
• postbud = mail carrier / postal worker

Postbud is a good general word because it is more neutral than postmand.

So in modern Danish, postbuddet is a very useful word to know.

The present tense in Danish, just like in English, can describe a general habit, routine, or usual rule.

So:

• Postbuddet ringer på, når pakken er for stor til postkassen.

does not necessarily mean it is happening right now. It means something like:

• The mail carrier rings the doorbell whenever the package is too big for the mailbox.

This is a very common use of the present tense in both Danish and English.

Possibly, depending on context. Ringer på most directly suggests ringing the bell, but in everyday translation it can sometimes be rendered more loosely as calls at the door or comes to the door.

If you want to be very literal:

• ringer på = rings the bell / rings at the door

If you want natural English in context, you might translate it as:

• The mail carrier comes to the door when the package is too big for the mailbox.

So the exact English wording can vary a little, even though the Danish phrase itself is straightforward.