Lastebilen stopper.

Because -en is the usual definite singular ending for many masculine/common-gender nouns in Norwegian.
So:

• en lastebil = a truck
• lastebilen = the truck

In this sentence, lastebilen is the subject and means the truck.

The dictionary form is lastebil.

You will often see it listed as:

• en lastebil = a truck

That tells you the noun takes en in the indefinite singular, and its definite singular form is normally lastebilen.

Yes. Norwegian very often forms compound nouns as a single word.

Here:

• laste- relates to cargo/load
• bil = car/vehicle

So lastebil is literally something like load-vehicle, i.e. truck.

This is very normal in Norwegian. Where English may use two words, Norwegian often uses one compound word.

Stopper is present tense.

The infinitive is:

• å stoppe = to stop

And the present tense is:

• stopper = stops / is stopping

So Lastebilen stopper can mean the truck stops or, depending on context, the truck is stopping.

Because stoppe is the infinitive form, while stopper is the present tense.

Compare:

• å stoppe = to stop
• stopper = stops / is stopping

This is a very common pattern for Norwegian verbs:

• å snakke → snakker
• å vente → venter
• å kjøre → kjører

So å stoppe becoming stopper is completely regular.

It can mean either, depending on context.

Norwegian often uses the simple present where English might use either:

• the truck stops
• the truck is stopping

Without extra context, Lastebilen stopper could describe:

• a general action
• something happening right now
• a sequence of events in a story

If you specifically want to emphasize an ongoing action, Norwegian can also use other constructions, but the simple present is very common.

Because Norwegian usually puts definiteness on the noun itself, instead of using a separate word like English the.

So English:

• the truck

Norwegian:

• lastebilen

The -en already carries the meaning of the.

Yes. This is the basic Norwegian sentence pattern:

• Subject + Verb

So:

• Lastebilen = subject
• stopper = verb

That gives a perfectly normal main clause: Lastebilen stopper.

Yes, in some cases, but not in this neutral statement.

Norwegian main clauses usually follow the V2 rule, meaning the finite verb is in the second position. In your sentence, the subject is first, so the verb comes second:

• Lastebilen stopper.

But if another element comes first, the verb still stays second:

• Nå stopper lastebilen. = Now the truck is stopping / stops now.

So the verb can appear before the subject, but only when something else takes the first position.

You would usually say:

• En lastebil stopper.

Here:

• en lastebil = a truck
• lastebilen = the truck

So the difference is:

• En lastebil stopper. = a truck stops
• Lastebilen stopper. = the truck stops

A rough guide for English speakers is:

• LAS-teh-bee-len

A few helpful notes:

• a in the first syllable is more like the a in father than in cat
• e in -te- is a short, clear vowel, something like eh
• i in bil is a long ee sound
• the final -en is usually light and unstressed

The stress is mainly on the first syllable: LAStebilen.

Yes. Å stoppe can be intransitive or transitive, depending on context.

In this sentence it is intransitive:

• Lastebilen stopper.
  The truck stops.

But it can also be transitive:

• Han stopper lastebilen.
  He stops the truck.

So Norwegian uses the same verb both for something stopping on its own and for someone stopping something.

Lastebil is normally a common-gender noun, which is why you get:

• en lastebil
• lastebilen

In Bokmål, common gender covers what is traditionally masculine, and many learners simply think of these nouns as en-words. That is usually the most practical approach at this stage.