Katten jakter på musen.

Because -en is the usual definite singular ending for many common-gender nouns in Bokmål.

• katt = cat
• katten = the cat

So instead of adding a separate word like English the, Norwegian often adds the definiteness to the end of the noun.

For the same reason: musen is the definite singular form, meaning the mouse.

• mus = mouse
• musen = the mouse

In Bokmål, mus can also sometimes be treated as feminine, in which case you may see musa. But musen is completely normal and standard.

Because Norwegian usually puts the onto the noun as an ending.

Compare:

• en katt = a cat

• katten = the cat

• en mus = a mouse
• musen = the mouse

This is one of the biggest differences from English noun structure.

Jakter is the present tense of å jakte.

• å jakte = to hunt
• jakter = hunt / is hunting

In Bokmål, present tense is often made by adding -r to the infinitive stem:

• å snakke → snakker
• å kjøpe → kjøper
• å jakte → jakter

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

• hunts
• is hunting

So Katten jakter på musen can correspond to English The cat hunts the mouse or The cat is hunting the mouse, depending on context.

You do not need an extra verb like is here.

Because jakte på is the normal expression for hunt/chase after something.

So you learn it as a unit:

• å jakte på noe/noen = to hunt something/someone

Even though på often means on, prepositions do not always match English directly. Here it is just the preposition that goes with jakte in this meaning.

Usually yes in many contexts, but literally it is closer to hunt after or be hunting/chasing.

That is why på appears. Norwegian often uses verb + preposition combinations where English uses just one verb.

Mainly from word order.

In a normal Norwegian main clause:

1. subject
2. verb
3. other elements

So in:

Katten jakter på musen

• Katten = subject
• jakter = verb
• på musen = what it is hunting

Norwegian does not rely much on noun case endings the way some other languages do, so word order is very important.

Yes. That would use the indefinite forms:

• en katt = a cat
• en mus = a mouse

So the difference is:

• Katten jakter på musen = the cat is hunting the mouse
• En katt jakter på en mus = a cat is hunting a mouse

In this sentence, they are being treated as common-gender nouns in Bokmål.

That is why you get:

• en katt → katten
• en mus → musen

A useful thing to remember is that Bokmål often groups masculine and feminine together as common gender in many learners’ materials, even though some nouns can also be feminine in more traditional or more colloquial usage.

Yes, in many varieties of Bokmål, mus can be feminine:

• ei mus = a mouse
• musa = the mouse

But many learners first learn the common-gender pattern:

• en mus
• musen

So musen is absolutely fine.

Most naturally, it sounds like a specific cat and a specific mouse, because both nouns are definite:

• katten = the cat
• musen = the mouse

So the sentence usually points to particular animals already known from context.

If you wanted a more general statement, you might choose a different wording depending on context.

A simple learner-friendly pronunciation guide would be:

Katten jakter på musen
KAHT-ten YAK-ter poh MOO-sen

A few notes:

• a in katt is short
• j in jakter sounds like English y
• u in musen is not exactly the English oo, but MOO-sen is a useful approximation for beginners

Yes. In modern Norwegian, the verb form does not change with the subject the way it does in English.

So you get:

• jeg jakter
• du jakter
• han jakter
• katten jakter

This is much simpler than English, where you have I hunt but he hunts.

Sometimes, but not freely.

The normal neutral order is:

Katten jakter på musen.

Norwegian main clauses usually follow the V2 rule, meaning the verb tends to stay in the second position. You can move another element to the front for emphasis, but then the verb still stays second:

• På musen jakter katten.
  This is possible, but marked and much less neutral.

For everyday use, stick with:

Katten jakter på musen.