Referansenummeret står i brevet.

Because Norwegian usually puts the definite article at the end of the noun.

• et referansenummer = a reference number
• referansenummeret = the reference number

The ending -et shows that the noun is definite and neuter.

This is very different from English, where you normally put the before the noun instead of attaching it to the end.

For the same reason: brevet is the definite form.

• et brev = a letter
• brevet = the letter

So i brevet means in the letter, not just in a letter.

In Norwegian, the definite article is often attached to the noun as a suffix.

So instead of:

• the reference number
• the letter

Norwegian says:

• referansenummeret
• brevet

That is the normal pattern for simple definite nouns.

This is a very common thing learners notice.

In Norwegian, stå can be used where English would often use be, especially when talking about information that is written somewhere.

So:

• Referansenummeret står i brevet.

sounds natural in Norwegian for something like “the reference number is stated / appears in the letter.”

Using er is not the usual choice here, because står gives the idea that the information is written there.

You will often see this with written information:

• Det står i boka.
• Det står på nettsiden.
• Navnet står på listen.

The basic verb stå means stand, but Norwegian often uses it more widely than English does.

Here it does not mean that the reference number is physically standing upright. Instead, it means something like:

• is written
• appears
• is stated

So this is an idiomatic, very normal use of stå with text and written information.

I is a preposition. Here it means in.

So:

• i brevet = in the letter

It is one of the most common Norwegian prepositions.

Yes. This is standard Norwegian main-clause word order:

• Referansenummeret = subject
• står = verb
• i brevet = prepositional phrase

So the pattern is basically:

Subject + verb + other information

That is the normal order in a simple statement.

Yes, that is possible, but it changes the emphasis.

• Referansenummeret står i brevet.
  This is the most neutral version.

• I brevet står referansenummeret.
  This puts more focus on where it is written.

When a Norwegian sentence begins with something other than the subject, the verb usually comes next. That is why it is:

• I brevet står referansenummeret not
• I brevet referansenummeret står

This is the V2 rule in Norwegian.

Both are neuter nouns.

You can tell from the indefinite article:

• et referansenummer
• et brev

Because they are neuter, their definite singular forms take -et:

• referansenummeret
• brevet

Yes. Norwegian often combines nouns into one word.

Here:

• referanse = reference
• nummer = number
• referansenummer = reference number

Then the definite ending is added:

• referansenummeret = the reference number

This is very typical in Norwegian. Where English often writes two words, Norwegian often uses one compound noun.

No, not in standard Norwegian for this meaning.

It should be written as one compound noun:

• referansenummer
• referansenummeret

Writing it as two words would usually be considered incorrect here.

This comes from usage, not just from literal word meanings.

In Norwegian, when people talk about information being in a document, message, website, article, or book, stå is very commonly used.

For example:

• Det står i e-posten.
• Adressen står på pakken.
• Svaret står i teksten.

So learners should get used to stå as a common verb for information that is written somewhere.

You would remove the definite endings:

• Et referansenummer står i et brev.

Grammatically, that means something like a reference number is in a letter, but it sounds much less natural in most real situations.

The original sentence is more natural because it refers to a specific reference number and a specific letter:

• referansenummeret
• brevet

So the definite forms make very good sense here.