Jeg kan ikke komme på etternavnet hennes.

Here komme på means to think of or to remember something suddenly.

So Jeg kan ikke komme på etternavnet hennes means something like:

• I can’t think of her last name
• I can’t remember her last name

This is an idiomatic expression in Norwegian. The verb komme by itself means come, but komme på has a different meaning here.

Because kan is a modal verb, meaning can.

In Norwegian, after a modal verb such as:

• kan = can
• vil = will
• må = must
• skal = shall / will

the next verb stays in the infinitive form, without å.

So:

• Jeg kan komme = I can come
• Jeg kan ikke komme på det = I can’t think of it

That is why you get:

• kan ikke komme not
• kan ikke kommer and not
• kan ikke å komme

In a main clause, Norwegian usually puts the finite verb in second position. Here the finite verb is kan.

So the basic order is:

• Jeg (subject)
• kan (finite verb)
• ikke (negation)
• komme på (infinitive verb phrase)
• etternavnet hennes (object)

This is very normal Norwegian word order.

Compare:

• Jeg kan komme. = I can come.
• Jeg kan ikke komme. = I can’t come.

Because the sentence is referring to a specific last name: her last name.

The noun is et etternavn = a last name / a surname.

It is a neuter noun, so:

• et etternavn = indefinite singular
• etternavnet = definite singular

When Norwegian uses a possessive after the noun, the noun is usually in the definite form:

• etternavnet hennes = her last name

So the -et ending is the definite ending for this neuter noun.

Norwegian often places possessives after the noun, and then the noun takes the definite form.

So:

• etternavnet hennes = her last name

This is one of the most common and natural patterns in everyday Norwegian.

You can also put the possessive before the noun:

• hennes etternavn

Both are possible, but etternavnet hennes is often the more neutral and natural choice in ordinary speech.

A rough comparison:

• etternavnet hennes = the usual everyday phrasing
• hennes etternavn = can sound more formal, contrastive, or emphasized depending on context

Because sin/sitt/sine is a reflexive possessive, used when the owner is the subject of the clause.

Here the subject is jeg (I), but the owner of the last name is her, not I.

So you need:

• hennes = her

Examples:

• Jeg liker bilen hennes. = I like her car.
• Hun liker bilen sin. = She likes her own car.

In your sentence:

• Jeg kan ikke komme på etternavnet hennes. means I can’t think of her last name, where her refers to some female person other than the subject jeg.

Yes. That is also correct, but the nuance is a little different.

• Jeg husker ikke etternavnet hennes = I don’t remember her last name
• Jeg kan ikke komme på etternavnet hennes = I can’t think of her last name

The second version often suggests:

• you feel like you almost know it
• it is on the tip of your tongue
• you are trying to recall it right now

So komme på is often more about retrieving something from memory in the moment.

Yes. Etternavn means surname / family name / last name.

English speakers often say last name, while British English may use surname more often. Norwegian etternavn covers that idea.

Related word:

• fornavn = first name / given name

So:

• fornavn = first name
• etternavn = last name

No. Like many verb + preposition combinations, komme på can have different meanings depending on context.

Common meanings include:

• think of / remember
• come up with
• sometimes other context-dependent uses

In this sentence, because the object is etternavnet hennes, the meaning is clearly remember / think of.

So learners should treat komme på as a fixed expression whose meaning depends on context, rather than translating it word-for-word as come on/to.

The most natural translations would usually be:

• I can’t think of her last name.
• I can’t remember her last name.

If you want to keep the exact nuance of trying to recall it right now, I can’t think of her last name is especially good.