De progressive brilleglas virker godt, bortset fra at læsefeltet er lille.

No. Here de is the, not they.

In de progressive brilleglas, de is the plural definite article used before an adjective. So the structure is:

• de = the
• progressive = progressive
• brilleglas = lenses

A useful pattern is:

• brilleglas = lenses / lens
• de progressive brilleglas = the progressive lenses

So in this sentence, de is not a pronoun.

Because the adjective is inflected.

The basic form is progressiv, but Danish adjectives often change form depending on grammar. After de and before a plural definite noun, the adjective usually takes -e:

• en progressiv løsning = a progressive solution
• de progressive brilleglas = the progressive lenses

So progressive is the expected form here.

Here it is plural, even though the form looks unchanged.

Some Danish nouns have the same form in singular and plural. Brilleglas is one of those. The grammar around it tells you the number:

• et brilleglas = a lens
• de progressive brilleglas = the progressive lenses

So de and the adjective form help show that this is plural.

Because virke can mean work, function, or be effective.

So virker godt means something like:

• works well
• functions well

This is very natural for products, solutions, medicines, glasses, technology, and so on.

Be careful: virke can also mean seem, depending on context. But here the meaning is clearly work/function well.

A close alternative would be fungerer godt, which is also natural.

Bortset fra means except for, apart from, or aside from.

It introduces an exception or one negative point after a generally positive statement.

So the sentence says the lenses work well, except for one thing.

You can use it with:

• a noun phrase: bortset fra prisen = except for the price
• a clause: bortset fra at læsefeltet er lille = except that the reading area is small

Because what follows is a full clause:

• læsefeltet er lille = the reading area is small

After bortset fra, Danish normally uses at before a clause, much like English except that.

So:

• bortset fra at læsefeltet er lille

Here at does not mean the infinitive marker to. It means something closer to that.

Because Danish forms compound nouns very freely.

læsefeltet is built from:

• læse = read / reading
• felt = field / area
• -et = the definite ending

So:

• et læsefelt = a reading area
• læsefeltet = the reading area

Writing it as one word is completely normal in Danish.

It marks the noun as definite: the reading area.

The base noun is:

• et læsefelt = a reading area

When it becomes definite, Danish usually adds the article to the end of the noun:

• læsefeltet = the reading area

This is a very common Danish pattern.

Because lille is irregular.

Many Danish adjectives take -t in the neuter singular:

• et stort felt = a big area
• feltet er stort = the area is big

But lille does not behave like that in normal modern Danish. You simply say:

• feltet er lille = the area is small

The form lidet exists, but it is old-fashioned or literary and would not be natural here.

Also note the plural form:

• små = small, plural
• felterne er små = the areas are small

Yes, it is a subordinate clause, even though you do not see much difference in this particular sentence.

The clause is:

• at læsefeltet er lille

Because there is no adverb like ikke, the word order looks similar to English. But it is still a subordinate clause.

You can see the difference more clearly in a sentence like:

• bortset fra at læsefeltet ikke er særlig stort

Notice how ikke comes before er in the subordinate clause.

So this sentence is useful for learning that at introduces a clause, not an infinitive.

Because brilleglas refers specifically to the lenses, while briller usually means the glasses as a whole.

That matters here because progressive really describes the lenses, and læsefeltet is a property of the lens design.

So:

• briller = glasses
• brilleglas = lenses

Using brilleglas is more precise in this sentence.

Yes, it is natural.

It sounds like a normal comment about progressive lenses. A speaker might also say:

• De progressive glas virker godt, bortset fra at læsefeltet er lille.
• De progressive brilleglas fungerer godt, men læsefeltet er lidt lille.

The original sentence is perfectly good Danish. It sounds clear, neutral, and fairly precise.