De progressive brilleglas ændrer synsfeltet.

It is capitalized only because it is the first word of the sentence.

Here, de is not the pronoun they. It is the plural definite article, meaning the.

You can tell because it is followed by an adjective and a noun:

de progressive brilleglas

So the structure is:

• de = the
• progressive = adjective
• brilleglas = noun

It is also not the old-fashioned formal De meaning you.

In Danish, adjectives usually take -e in:

• the plural
• definite noun phrases

This phrase is both plural and definite, so progressive becomes progressive with final -e.

Compare:

• et progressivt brilleglas = one progressive lens
• progressive brilleglas = progressive lenses
• de progressive brilleglas = the progressive lenses

So the -e ending is normal adjective agreement.

Because Danish normally writes compound nouns as a single word.

So:

• brille
  • glas = brilleglas
• syn
  • felt = synsfelt

This is very common in Danish. English sometimes uses two words where Danish prefers one compound word.

Some Danish nouns have the same form in singular and plural, especially some neuter nouns.

So:

• et brilleglas = one lens
• to brilleglas = two lenses

The number is understood from the article or from context. In this sentence, de shows that the noun phrase is plural and definite.

When a Danish noun is definite and has an adjective before it, Danish normally shows definiteness with den / det / de before the adjective, and the noun itself stays in its ordinary plural form.

So:

• de progressive brilleglas = correct

This is a very important pattern in Danish.

A simple way to think of it is:

• article + adjective + noun

rather than putting the definite marking on the noun in this kind of phrase.

The ending -et is the definite singular ending for a neuter noun.

Base form:

• et synsfelt = a field of vision

Definite form:

• synsfeltet = the field of vision

Unlike English, Danish often puts the on the end of the noun instead of using a separate word.

Ændrer is the present tense of ændre.

• at ændre = to change
• ændrer = changes / change

Danish verbs do not change for person the way English verbs sometimes do. The same present-tense form is used with all subjects:

• jeg ændrer
• du ændrer
• de ændrer

So here ændrer simply means present tense.

It is the normal Danish main-clause order:

• De progressive brilleglas = subject
• ændrer = finite verb
• synsfeltet = object

So the pattern is:

subject + verb + object

This is the most basic sentence order in Danish.

Yes, a few:

• æ in ændrer has a vowel somewhat like the a in cat, though not exactly the same.
• y in synsfeltet has no direct English equivalent. A useful trick is to shape your lips as if saying oo, but try to say ee.
• Danish d is often softer than English speakers expect, especially inside words.

So even if you understand the grammar, the pronunciation may still feel difficult at first. That is completely normal in Danish.