Jeg glemte etternavnet hennes et øyeblikk, men så kom jeg på både fornavnet og fødselsdatoen.

Glemte is the past tense of å glemme, which means to forget.

• å glemme = to forget
• jeg glemmer = I forget / I am forgetting
• jeg glemte = I forgot

So Jeg glemte etternavnet hennes means I forgot her last name.

Both are possible, but etternavnet hennes is the most common and natural way to say her last name in Norwegian.

Norwegian often uses this pattern:

• noun + definite ending + possessive

So:

• etternavnet hennes = her last name
• boka mi = my book
• vennen hans = his friend

You can also say hennes etternavn, but that usually sounds a bit more formal, emphatic, or literary.

They all have the definite form because the sentence refers to specific information about a particular person:

• etternavnet hennes = her last name
• fornavnet = the first name
• fødselsdatoen = the birth date

In English, we often use a possessive or context without thinking about definiteness much, but Norwegian often marks the noun itself as definite.

Forms here:

• etternavn → etternavnet
• fornavn → fornavnet
• fødselsdato → fødselsdatoen

Et øyeblikk literally means a moment and here it means for a moment or briefly.

So:

• Jeg glemte etternavnet hennes et øyeblikk
  = I forgot her last name for a moment

Norwegian often uses et øyeblikk without adding a separate word for for in this kind of expression.

Here kom på is part of the verb å komme på, which often means to remember, to think of, or to occur to someone.

So:

• så kom jeg på ... = then I remembered ...

This is an idiomatic expression. You should learn å komme på as a unit.

Examples:

• Jeg kom på navnet hennes. = I remembered her name.
• Nå kom jeg på noe viktig. = I just thought of something important.

So even though kom by itself means came, kom på has a different meaning.

Because Norwegian main clauses follow the V2 rule: the finite verb usually comes in the second position.

In men så kom jeg på ...:

• men = but
• så = then
• kom = verb
• jeg = subject

The clause itself is så kom jeg på ..., where så takes the first position, so the verb kom must come next.

That is why you get:

• så kom jeg på ...

not

• så jeg kom på ...

This kind of inversion is very common in Norwegian after adverbs like så, derfor, da, nå, etc.

Men så means but then.

• men = but
• så = then

It shows a contrast followed by the next event:

• first: I forgot her last name for a moment
• then: I remembered both her first name and her birth date

So men så helps connect the two ideas naturally.

Både ... og ... means both ... and ....

In the sentence:

• både fornavnet og fødselsdatoen
  = both the first name and the birth date

This is a very common structure in Norwegian:

• Jeg liker både te og kaffe. = I like both tea and coffee.
• Hun kan både synge og danse. = She can both sing and dance.

So here it emphasizes that the speaker remembered two things, not just one.

Because Norwegian nouns have different grammatical genders, and the definite ending depends on the gender.

Here:

• et fornavn = a first name → neuter noun
  definite: fornavnet
• en fødselsdato = a birth date → common gender noun
  definite: fødselsdatoen

A quick summary:

• en nouns usually become -en in the definite singular
• et nouns usually become -et in the definite singular

So:

• etternavn → etternavnet
• fornavn → fornavnet
• fødselsdato → fødselsdatoen

You usually have to learn the gender of each noun along with the noun itself.

In this sentence:

• et etternavn = a last name
• et fornavn = a first name
• et øyeblikk = a moment
• en fødselsdato = a birth date

A very good habit is to memorize nouns with their article:

• et etternavn
• et fornavn
• en fødselsdato

That makes it much easier to form the definite versions later.

Yes. Once the person is already clear from context, Norwegian often does not repeat the possessive if it is obvious.

So after etternavnet hennes, it is understood that fornavnet and fødselsdatoen are also hers.

A fuller version could be:

• ... men så kom jeg på både fornavnet hennes og fødselsdatoen hennes.

That is grammatically possible, but it sounds heavier and more repetitive. The original sentence is more natural.

In this sentence, because of et øyeblikk and the following men så kom jeg på, it clearly means the speaker temporarily couldn’t remember it.

So the idea is:

• I blanked on her last name for a moment
• but then I remembered...

Norwegian å glemme can mean both to forget completely and to fail to remember for the moment, depending on context. Here the context makes it temporary.