Enkeltstyrkeglassene er i brillestellet.

Because Danish usually writes compound nouns as one word.

Here, enkeltstyrkeglassene is built from:

• enkeltstyrke = single vision / single prescription strength
• glas = lens
• -ene = definite plural ending

So Danish packs the whole idea into one compound noun, where English would normally use several words.

The dictionary form is glas.

A useful thing to know is that glas can be both singular and plural in the indefinite form:

• et glas = a glass / a lens
• glas = glasses / lenses

Then the definite plural becomes glassene, meaning the lenses.

Because Danish often puts the at the end of the noun instead of using a separate word before it.

So:

• glassene = the lenses
• brillestellet = the frame

A separate article such as de or det is mainly used in other structures, for example when there is an adjective before the noun:

• de nye glas = the new lenses
• det nye brillestel = the new frame

They show different grammatical information.

• -ene on glassene marks definite plural
• -et on brillestellet marks definite singular for a neuter noun

So the sentence has:

• the lenses = plural
• the frame = singular

Yes. In an eyewear context, glas usually means an optical lens, not a drinking glass.

So in this sentence, glassene means the lenses in a pair of glasses. Context tells you which meaning is intended.

It means the eyeglass frame or the spectacle frame.

It is made from:

• brille = glasses / eyeglasses
• stel = frame, mounting, or setting
• -et = definite singular ending

Also, in Danish compounds, the first noun is often in a basic singular form, so you get brillestel, not something like brillerstel.

Because the lenses are understood as being physically in the frame.

So i brillestellet means they are inside it or mounted in it. That is the natural preposition for this relationship.

Yes.

• er i is neutral and simply states where they are
• sidder i can sound a bit more physical or idiomatic, as if the lenses are fitted into the frame

So both can work, but sidder i may sound more natural in some real-life optics contexts.

Not really. It is a very standard main-clause pattern:

Enkeltstyrkeglassene + er + i brillestellet

That is:

• subject
• verb
• prepositional phrase

This matches English quite closely in this sentence. More generally, Danish main clauses follow the verb-second rule, and here the subject comes first, so the sentence looks straightforward.