Bekreftelsen kommer dersom bostedsattesten er klar.

Because bekreftelsen is the definite singular form of bekreftelse.

• en bekreftelse = a confirmation
• bekreftelsen = the confirmation

Norwegian usually adds the definite article to the end of the noun instead of putting a separate word in front, as English does.

In Norwegian, the present tense is very often used for future meaning when the context makes the future clear.

So:

• Bekreftelsen kommer literally looks like The confirmation comes
• but it often means The confirmation will come

You could also say vil komme, but kommer is very natural here.

Dersom means if.

It is very close in meaning to hvis, which also means if. The main difference is style:

• dersom = a bit more formal or written
• hvis = very common in everyday speech and writing

So this sentence could also be:

• Bekreftelsen kommer hvis bostedsattesten er klar.

Both are correct.

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

It can be broken down like this:

• bosted = place of residence
• attest = certificate
• bostedsattest = certificate of residence
• bostedsattesten = the certificate of residence

The -s- in the middle is a linking sound often used in compounds. English learners often need to get used to long Norwegian compound words like this.

Because klar is a predicative adjective here, and it agrees with the noun.

In this sentence, bostedsattesten is:

• singular
• common gender

So the adjective is:

• klar = ready

Compare:

• Attesten er klar. = The certificate is ready.
• Dokumentet er klart. = The document is ready.
  
  • neuter singular, so klart
• Dokumentene er klare. = The documents are ready.
  
  • plural, so klare

Because dersom introduces a subordinate clause.

In Norwegian subordinate clauses, the word order is usually more straightforward than in main clauses:

• dersom bostedsattesten er klar
  = if the certificate of residence is ready

Here:

• bostedsattesten = subject
• er = verb
• klar = adjective/complement

That is normal subordinate-clause word order.

Yes. You can put the if-clause first:

• Dersom bostedsattesten er klar, kommer bekreftelsen.

This means the same thing.

But notice the word order in the main clause after the comma:

• kommer bekreftelsen
• not bekreftelsen kommer

That happens because Norwegian uses verb-second word order in main clauses. When the sentence starts with something other than the subject, the verb comes before the subject.

For the same reason as with bekreftelsen: Norwegian usually marks definiteness at the end of the noun.

So:

• en bostedsattest = a certificate of residence
• bostedsattesten = the certificate of residence

English uses a separate article (the), but Norwegian often attaches it to the noun.

No, not in this sentence.

Although klar can sometimes mean clear, here it means ready or available/prepared.

So bostedsattesten er klar means something like:

• the certificate of residence is ready
• the certificate of residence has been prepared

The exact English wording depends on context.

Yes, it sounds fairly formal or administrative.

Clues include:

• dersom instead of the more everyday hvis
• nouns like bekreftelsen and bostedsattesten, which sound bureaucratic or official

This is the kind of sentence you might see in a letter, email, or government message.