Klageren bruker partsinnsynet under realitetsbehandlingen.

Because klageren is the definite singular form of en klager.

• en klager = a complainant / an appellant
• klageren = the complainant / the appellant

In Norwegian, the definite article is usually added to the end of the noun instead of being a separate word like the in English.

Norwegian usually puts the at the end of the noun as a suffix.

So:

• klageren = the complainant
• partsinnsynet = the party access / the right of access
• realitetsbehandlingen = the substantive consideration / the merits processing

This is very normal in Norwegian. English uses a separate word (the), but Norwegian often uses an ending instead.

Bruker is the present tense of å bruke.

• å bruke = to use
• bruker = uses / is using

So Klageren bruker ... means The complainant uses ...

Norwegian present tense is quite simple compared with English. The verb does not change depending on the subject:

• jeg bruker = I use
• du bruker = you use
• han bruker = he uses
• de bruker = they use

Because Norwegian usually writes compound nouns as one word.

Partsinnsynet is built from:

• part = party, especially a party in a legal/administrative case
• innsyn = access to documents / inspection / insight
• -et = definite singular ending

So partsinnsynet means something like the party’s right of access to case documents or the access right for a party.

English often writes these ideas as separate words, but Norwegian usually combines them into a single compound noun.

That -s- is a very common linking sound/letter in Norwegian compounds.

Here:

• part
  • innsyn becomes partsinnsyn

This -s- does not usually have a separate meaning by itself. It just helps connect the parts of the compound. English learners often notice this because English does not use this pattern in quite the same way.

Grammatically, it is another compound noun in the definite singular.

It is built from:

• realitet = substance / merits
• behandling = treatment / handling / processing
• -en = definite singular

So:

• en realitetsbehandling = a substantive consideration / a consideration on the merits
• realitetsbehandlingen = the substantive consideration / the merits stage

In legal or administrative language, this usually refers to dealing with the actual substance of the case, not just procedural issues.

No, here under means during.

So:

• under realitetsbehandlingen = during the substantive consideration of the case

Norwegian under can mean:

• physically under
• temporally during

In this sentence, it is clearly the time-related meaning: during the merits stage / during the substantive handling.

Because under is a natural way to say during an ongoing process or stage.

• under realitetsbehandlingen = during the substantive processing
• i realitetsbehandlingen would sound less natural here in most legal/administrative contexts

So this choice is mostly about normal usage and style, especially formal or legal Norwegian.

Yes. The sentence has a very standard Norwegian word order:

• Klageren = subject
• bruker = verb
• partsinnsynet = object
• under realitetsbehandlingen = adverbial phrase

So the pattern is:

Subject + Verb + Object + Adverbial

That is the normal word order in a main clause when nothing else has been moved to the front.

Norwegian follows the V2 rule in main clauses, which means the verb usually stays in the second position.

For example:

• Under realitetsbehandlingen bruker klageren partsinnsynet.

Here Under realitetsbehandlingen comes first, so the verb bruker must still come second, before the subject klageren.

That word order can feel unfamiliar to English speakers, so it is a very common thing to ask about.

From the forms here, you can tell the genders:

• en klager → common gender
• et partsinnsyn → neuter
• en realitetsbehandling → common gender

That is why the definite forms look like this:

• klageren
• partsinnsynet
• realitetsbehandlingen

The endings help you identify both definiteness and gender.

Yes.

• klager can mean a complainant / complainants, depending on context
• klageren specifically means the complainant

Also, klager can be the present tense of the verb å klage:

• han klager = he complains

So context matters. In your sentence, klageren is clearly the noun the complainant.

Not really. It is grammatical and natural, but it sounds formal and legal/administrative.

Words like:

• partsinnsyn
• realitetsbehandling

belong more to official language than everyday conversation. A native speaker would understand that this is from a legal or bureaucratic context rather than ordinary daily speech.