Brilleglassene er gode, bortset fra at antirefleksbehandlingen er dyr.

It refers to the lenses in the glasses, not the whole pair of glasses.

• brillerne = the glasses
• brilleglassene = the lenses

So this sentence is specifically talking about the quality of the lenses.

Because it is a definite plural noun: the lenses.

A useful pattern is:

• et brilleglas = a lens
• brilleglasset = the lens
• brilleglas = lenses
• brilleglassene = the lenses

The -ene ending is a very common way to make nouns definite plural in Danish. Because the word already ends in s, the spelling shows ss before the ending.

Because Danish normally writes compound nouns as one word.

So:

• brille = glasses / spectacles
• glas = glass / lens
• brilleglas = spectacle lens

This is very normal in Danish. English often uses separate words where Danish prefers one compound word.

Because gode agrees with a plural subject.

The subject is brilleglassene, which is plural, so the adjective must also be plural:

• glasset er godt = the lens is good
• brilleglassene er gode = the lenses are good

In Danish, adjectives after er still agree with the noun. That is different from English, where good stays the same.

It means except that or apart from the fact that.

It introduces an exception to the positive statement:

• Brilleglassene er gode = The lenses are good
• bortset fra at antirefleksbehandlingen er dyr = except that the anti-reflective coating is expensive

You can think of bortset fra at as a fixed expression.

Because what follows is a full clause:

• antirefleksbehandlingen er dyr

When Danish follows bortset fra with a whole clause, at is used.

Compare:

• bortset fra prisen = except for the price
  
  • here it is only a noun phrase, so no at
• bortset fra at prisen er høj = except that the price is high
  
  • here it is a full clause, so at is needed

It is another Danish compound noun:

• anti-
• refleks
• behandling

Together they make antirefleksbehandling = anti-reflective treatment / coating.

The ending -en makes it definite singular:

• en antirefleksbehandling = an anti-reflective treatment
• antirefleksbehandlingen = the anti-reflective treatment

This also tells you that behandling is a common-gender noun.

Because antirefleksbehandlingen is singular and common gender, so the adjective takes the base form dyr.

Compare:

• en behandling er dyr = a treatment is expensive
• et glas er dyrt = a lens is expensive if the noun is neuter singular
• behandlingerne er dyre = the treatments are expensive

So in this sentence, dyr is the correct agreement form.

Because at introduces a subordinate clause.

In a clause like this, Danish normally keeps subject + verb order:

• at antirefleksbehandlingen er dyr

A useful thing to notice is that sentence adverbs come before the verb in subordinate clauses:

• at antirefleksbehandlingen ikke er dyr = that the anti-reflective coating is not expensive

So this part of the sentence follows normal subordinate-clause word order.

Yes, but the meaning changes.

• Brillerne er gode = The glasses are good
  
  • this refers to the whole glasses
• Brilleglassene er gode = The lenses are good
  
  • this focuses only on the lenses

So the original sentence is more specific.

The comma separates the main statement from the added exception:

• Brilleglassene er gode
• bortset fra at antirefleksbehandlingen er dyr

This kind of comma is very natural in Danish when adding extra information or an exception. You may also see some variation in comma use depending on writing style, but the comma here looks perfectly normal.