Kontaktlinsen passer så lenge hornhinnen ikke endres.

Because Norwegian usually writes compound nouns as a single word.
So kontaktlinse = contact lens.

When it becomes definite singular, Norwegian adds the article as a suffix:

• en kontaktlinse = a contact lens
• kontaktlinsen = the contact lens

So kontaktlinsen is not two words in Norwegian.

Kontaktlinsen is the definite singular form, meaning the contact lens.

Norwegian often puts the definite article at the end of the noun:

• en linse = a lens
• linsen = the lens

Here, the speaker is probably referring to a specific contact lens that has already been mentioned, or to the lens being discussed in a particular medical/optical context.

For the same reason: hornhinnen means the cornea.

Forms:

• en hornhinne = a cornea
• hornhinnen = the cornea

In medical or technical language, Norwegian often uses the definite form when talking about a specific body part that is understood from the context.

Here passer means something like fits, is suitable, or continues to fit properly.

The verb is å passe, which can have several meanings depending on context, such as:

• to fit
• to suit
• to watch / look after
• to be careful

In this sentence, it is clearly the fit meaning: the contact lens fits properly.

Så lenge means as long as in the sense of provided that or for as long as.

In this sentence, it introduces a condition:

• Kontaktlinsen passer så lenge hornhinnen ikke endres.
• The contact lens fits as long as the cornea does not change.

So the idea is: the lens will keep fitting if the cornea stays the same.

Because hornhinnen ikke endres is a subordinate clause, introduced by så lenge.

In Norwegian subordinate clauses, the usual word order puts ikke before the verb:

• Main clause: Hornhinnen endres ikke.
• Subordinate clause: ... så lenge hornhinnen ikke endres.

So this is normal subordinate-clause word order.

Yes. Endres is the passive form of endre.

• å endre = to change something
• å endres = to be changed / to change

In this sentence, hornhinnen ikke endres means the cornea is not changed or more naturally in English, the cornea does not change.

Norwegian often uses the -s passive where English might simply use an active intransitive verb.

Yes, you could.

• ikke endres
• ikke blir endret

Both can mean is not changed / does not change, but endres is more compact and often sounds more natural in formal or technical statements like this one.

So the original version is very natural.

Most naturally, it sounds like it is talking about one specific lens or the lens currently being discussed.

That said, Norwegian sometimes uses the singular in a fairly general way too, especially in technical explanations. Context decides.

So depending on the situation, it could mean:

• a particular fitted lens
• the contact lens as a type in the situation being discussed

But without extra context, the contact lens is the safest reading.

Because Norwegian usually expresses definiteness by attaching the article to the noun.

Examples:

• en kontaktlinse = a contact lens

• kontaktlinsen = the contact lens

• en hornhinne = a cornea
• hornhinnen = the cornea

So instead of a separate word like English the, Norwegian often uses an ending such as -en, -a, or -et, depending on the noun.

The sentence has:

• a main clause: Kontaktlinsen passer
• a subordinate clause: så lenge hornhinnen ikke endres

So structurally it is:

• [Subject] + [Verb] + [subordinate clause]

More specifically:

• Kontaktlinsen = subject
• passer = verb
• så lenge hornhinnen ikke endres = conditional clause

That is why the sentence is quite compact but still very typical Norwegian.

Yes, that is possible, but it is not exactly the same nuance.

• så lenge = as long as
• hvis = if

Så lenge emphasizes duration or continued validity: the lens keeps fitting for as long as the cornea stays unchanged.

Hvis sounds more like a simple condition.

So så lenge is the better choice here if the idea is ongoing fit over time.