Vognkortet og leasingavtalen dokumenterer kjøretøyet.

Those endings mark the definite singular, the equivalent of the in English.

• vognkortet = the vehicle registration document
• leasingavtalen = the leasing agreement
• kjøretøyet = the vehicle

In Norwegian, definiteness is usually added to the end of the noun instead of using a separate word like the.

Very roughly:

• et vognkort = a registration document

• vognkortet = the registration document

• en leasingavtale = a leasing agreement

• leasingavtalen = the leasing agreement

• et kjøretøy = a vehicle
• kjøretøyet = the vehicle

Because Norwegian usually expresses the by attaching it to the noun itself.

So instead of saying something like the vehicle, Norwegian often says:

• kjøretøyet

This is one of the biggest structural differences from English. The definiteness is built into the noun ending.

Vognkort is the official vehicle document/registration paper for a car or other vehicle. In practical English, it is often translated as:

• the vehicle registration document
• the registration certificate
• sometimes simply the registration papers

So vognkortet means the registration document.

Yes. Norwegian very often combines related words into one compound noun.

Here:

• leasing = leasing
• avtale = agreement

Together:

• leasingavtale = leasing agreement
• leasingavtalen = the leasing agreement

This is very normal in Norwegian. English often writes these as two words, but Norwegian frequently writes them as one.

Because in Norwegian compounds, the last part is the main noun, and that is the part that takes the definite ending.

Here the head noun is:

• avtale = agreement

So:

• leasingavtale = leasing agreement
• leasingavtalen = the leasing agreement

The first part, leasing-, just describes the type of agreement.

Dokumenterer is the present tense of dokumentere = to document.

So:

• dokumentere = to document
• dokumenterer = document / documents / is documenting, depending on context

In this sentence, it means:

• document
• or more naturally in English, serve as documentation for / prove the details of

Because Norwegian verbs do not change form depending on whether the subject is singular or plural.

So Norwegian uses the same present-tense form with:

• jeg dokumenterer = I document
• du dokumenterer = you document
• han dokumenterer = he documents
• de dokumenterer = they document

That is different from English, where we say document but documents.

So even though vognkortet og leasingavtalen is a plural subject, dokumenterer stays the same.

This sentence uses normal subject–verb–object order:

• Vognkortet og leasingavtalen = subject
• dokumenterer = verb
• kjøretøyet = object

So the structure is basically:

• The registration document and the leasing agreement document the vehicle

This is the most basic Norwegian sentence pattern.

Because the sentence is talking about a specific vehicle, not just any vehicle.

• kjøretøy = a vehicle / vehicle
• kjøretøyet = the vehicle

So the sentence is not saying that these documents document vehicles in general. It means they document the particular vehicle in question.

They are:

• et vognkort → neuter
• en leasingavtale → common gender in standard Bokmål usage here
• et kjøretøy → neuter

That is why the definite forms look like this:

• vognkortet
• leasingavtalen
• kjøretøyet

A quick rule of thumb:

• many neuter nouns take -et in the definite singular
• many common gender nouns take -en in the definite singular

Yes. In Bokmål, many nouns can be either common gender or specifically feminine, depending on style and preference.

So you may see:

• en avtale / avtalen
• or ei avtale / avtala

In this sentence, leasingavtalen uses the very common -en form, which is completely standard.

Yes. Og means and.

Here it links two nouns:

• vognkortet og leasingavtalen
• the registration document and the leasing agreement

So both of those together are the subject of the sentence.

A possible indefinite version would be:

• Et vognkort og en leasingavtale dokumenterer et kjøretøy.

That would mean something like:

• A registration document and a leasing agreement document a vehicle.

Compare that with the original definite version:

• Vognkortet og leasingavtalen dokumenterer kjøretøyet.
• The registration document and the leasing agreement document the vehicle.

So the endings are doing a lot of important work.