Underarmen er sliten.

Because underarmen is the definite singular form of en underarm.

• en underarm = a forearm
• underarmen = the forearm

In Norwegian, body parts are very often used in the definite form when it is clear whose body part is meant. So Underarmen er sliten can sound natural even without saying my.

It can, but it often does not need to.

Norwegian frequently uses the definite form for body parts when the owner is obvious from context. So instead of saying my forearm, Norwegian often just says the forearm.

Compare:

• Underarmen er sliten. = The forearm is tired / My forearm is tired
• Underarmen min er sliten. = My forearm is tired

The version with min is more explicit and can sound more emphatic.

It is made up of:

• under- = under / lower
• arm = arm
• -en = the definite ending for a common-gender singular noun

So underarmen literally looks like the lower arm.

One important note: this is a false friend for English speakers. Norwegian underarm means forearm, not English underarm in the sense of armpit.

Because Norwegian uses å være (to be) with adjectives like sliten.

• Underarmen er sliten. = The forearm is tired.

Using har here would be wrong if you want to describe the forearm with an adjective.

You use ha (to have) with nouns or certain set expressions, for example:

• Jeg har vondt i underarmen. = I have pain in my forearm.
• Jeg har en sliten underarm. = I have a tired forearm.

But in the original sentence, the structure is simply:

• noun + er + adjective

Because adjectives in Norwegian agree with the noun.

Underarmen is a singular common-gender noun, so the adjective takes the form sliten.

Compare:

• en underarm → sliten
• et kne → slitent
• underarmene → slitne

So:

• Underarmen er sliten. = correct
• Kneet er slitent. = correct
• Armene er slitne. = correct

In Bokmål, it is normally treated as a common-gender noun, so you usually learn it as:

• en underarm
• underarmen

For a beginner, the safest approach is to use en and the definite form -en.

Yes. Sliten can describe a person, a body part, or even an object, depending on context.

Examples:

• Jeg er sliten. = I am tired.
• Beina er slitne. = The legs are tired.
• Skoene er slitne. = The shoes are worn out.

With a body part, sliten usually means it feels fatigued, overworked, or strained, not necessarily emotionally tired.

It can be natural, but it depends on exactly what you mean.

If you mean the forearm feels fatigued from use, Underarmen er sliten works.

But Norwegian also often uses other patterns:

• Jeg er sliten i underarmen. = I am tired in the forearm / My forearm feels tired.
• Jeg har vondt i underarmen. = My forearm hurts.

So if the intended meaning is more like pain, then vondt is more natural than sliten.

The pattern is:

• Underarmen = subject
• er = verb
• sliten = adjective/predicative

So this is a very standard Norwegian sentence structure:

subject + verb + adjective

Like:

• Huset er stort. = The house is big.
• Boka er interessant. = The book is interesting.
• Underarmen er sliten. = The forearm is tired.

Yes. In a yes/no question, the verb usually comes first:

• Er underarmen sliten? = Is the forearm tired?

That is the normal inversion pattern in Norwegian.

You would change both the noun and the adjective:

• Underarmene er slitne. = The forearms are tired.

Here:

• underarmene = the forearms
• slitne = tired (plural form)

So the adjective changes to match the plural noun.