Avstøpningen var ubehagelig. Ikke desto mindre går tilvenningen til høreapparatet bra.

Because avstøpningen is the definite singular form of en avstøpning.

• en avstøpning = a cast / an impression
• avstøpningen = the cast / the impression

The ending -en is one common way Norwegian marks the on a noun.

Because ubehagelig comes after var, so it is a predicate adjective, not an adjective directly in front of the noun.

Compare:

• den ubehagelige avstøpningen = the unpleasant cast
  
  • here the adjective is attributive, so it changes form
• avstøpningen var ubehagelig = the cast was unpleasant
  
  • here the adjective is predicative, so definiteness does not affect it in the same way

So the sentence is correct as written.

Ikke desto mindre means nevertheless, nonetheless, or all the same.

It is a fixed expression and sounds a bit more formal or written than everyday speech. In casual Norwegian, people very often use likevel instead.

So:

• Ikke desto mindre = more formal/written
• Likevel = very common in everyday use

This is because Norwegian follows the V2 rule in main clauses: the finite verb usually comes in the second position.

Basic order:

• Tilvenningen til høreapparatet går bra.

But if you move an expression like Ikke desto mindre to the front, the verb must still stay second:

• Ikke desto mindre går tilvenningen til høreapparatet bra.

So går comes before tilvenningen because of normal Norwegian word order after fronting.

Yes. Tilvenningen is still the subject.

In English, the subject usually stays before the verb, but in Norwegian it can come after the verb when something else is placed first in the sentence.

So in:

• Ikke desto mindre går tilvenningen til høreapparatet bra

the parts are:

• Ikke desto mindre = sentence adverbial
• går = finite verb
• tilvenningen til høreapparatet = subject

Tilvenningen is the definite singular form of tilvenning.

• tilvenning = adjustment / acclimatization / getting used to something
• tilvenningen = the adjustment / the process of getting used to it

It is a noun related to the idea of getting used to something. In everyday Norwegian, learners will often also meet the verb expression:

• å venne seg til noe = to get used to something

So tilvenning is a noun built from that same general idea.

Because the noun tilvenning normally takes til when you say what someone is getting used to.

So:

• tilvenning til noe = adjustment to something
• tilvenningen til høreapparatet = the adjustment to the hearing aid

The second til is not random; it belongs naturally with tilvenning.

Høreapparatet is the definite singular form:

• et høreapparat = a hearing aid
• høreapparatet = the hearing aid

The definite form is used because the hearing aid is understood as a specific one already known from the context, not just any hearing aid.

This also shows that høreapparat is a neuter noun:

• et høreapparat
• høreapparatet

Går bra is a very natural Norwegian expression for saying that a process, situation, or activity is going well.

So:

• Tilvenningen går bra = The adjustment is going well

You could say er bra in some contexts, but it would mean something slightly different. Er bra describes something as good; går bra focuses on how it is progressing.

Compare:

• Filmen er bra = the film is good
• Operasjonen gikk bra = the operation went well
• Tilvenningen går bra = the adjustment is going well

So går bra is the better choice here.