Optikeren måler pupillavstanden for brilleglassene.

Because -en is the definite singular ending for many masculine nouns in Norwegian.

• en optiker = an optician
• optikeren = the optician

Norwegian usually marks definiteness by adding an ending to the noun, instead of using a separate word like English the.

Måler is the present tense of å måle.

• å måle = the infinitive, to measure
• måler = present tense, measures / is measuring

In Norwegian, the same present-tense form is used for all persons:

• jeg måler
• du måler
• optikeren måler

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

• measures
• is measuring

So the exact English choice depends on context. The Norwegian form itself does not change.

Because Norwegian normally writes compound nouns as one word.

Here, pupillavstanden is built from:

• pupill = pupil
• avstand = distance
• -en = definite singular

So the structure is basically:

pupill + avstand + en
= the pupil distance

This is very normal in Norwegian. English often uses several words where Norwegian prefers one compound.

Because the whole compound is in the definite singular.

The main noun in the compound is the last part, avstand. That is the part that gets the definite ending:

• avstand = distance
• avstanden = the distance

Then when it becomes part of the compound:

• pupillavstand = pupil distance
• pupillavstanden = the pupil distance

In Norwegian compounds, the last element controls the grammatical form.

Here for means for in the sense of for use with or for the purpose of.

So the idea is that the optician measures the pupil distance for the lenses, meaning so the lenses can be made or positioned correctly.

It does not mean because of here. It is about purpose or intended use.

Because brilleglassene refers specifically to the lenses, not the glasses as a whole.

• briller / brillene = glasses, spectacles
• brilleglass / brilleglassene = eyeglass lens / the eyeglass lenses

That is more precise, because pupil distance is especially relevant to where the lenses are placed.

It is another compound noun:

• brille = glasses / spectacles
• glass = glass, and in this context lens
• -ene = definite plural

So:

brille + glass + ene
= brilleglassene
= the eyeglass lenses

This is a good example of how Norwegian builds very specific nouns by combining smaller words.

No, this is a very standard main-clause pattern:

Subject + verb + object/complement

• Optikeren = subject
• måler = verb
• pupillavstanden for brilleglassene = what is being measured / related phrase

Norwegian is a V2 language, which means the finite verb normally comes second in main clauses. Here that happens naturally because the subject comes first.

Yes, this is standard Bokmål Norwegian.

A learner using Bokmål can treat this as a normal, natural sentence. Some forms might look different in Nynorsk, but nothing here is unusual for Bokmål.