Optikeren forklarer langsynetheden.

Because Danish usually adds the definite article to the end of the noun.

• optiker = optician
• optikeren = the optician

This is very common in Danish. Instead of a separate word like English the, Danish often uses a suffix:

• en stol = a chair
• stolen = the chair

Here, optikeren refers to a specific optician, not just any optician.

For the same reason: the noun is in the definite form.

• langsynethed = farsightedness
• langsynetheden = the farsightedness

The ending -en marks definiteness here too. So Danish is saying not just the general idea of farsightedness, but a specific case or condition being discussed.

Because Danish often uses a suffixed definite article instead of a separate word.

So English:

• the optician
• the farsightedness

becomes Danish:

• optiker-en
• langsynethed-en

Danish does also have separate definite words like den, det, and de, but those are mainly used in other structures, such as when there is an adjective:

• optikeren = the optician
• den dygtige optiker = the skilled optician

forklarer is the present tense of forklare.

A useful thing for English speakers: Danish verbs do not change for person the way English verbs sometimes do.

So:

• jeg forklarer = I explain / am explaining
• du forklarer = you explain
• han forklarer = he explains
• optikeren forklarer = the optician explains

The form forklarer stays the same no matter who the subject is.

This is the normal word order for a simple main clause:

subject + verb + object

• Optikeren = subject
• forklarer = verb
• langsynetheden = object

So the sentence structure is very similar to English: The optician explains the farsightedness.

A very important Danish rule is that the finite verb is usually in second position in main clauses. In this sentence, the subject comes first, so the verb naturally comes second.

You can tell from both position and meaning.

In this sentence:

• Optikeren comes first, before the verb, so it is the subject.
• forklarer is the action.
• langsynetheden comes after the verb and receives the action, so it is the object.

Danish does not mark subjects and objects with case endings the way some languages do, so word order matters a lot.

It is a longer derived noun made from several parts:

• langsynet = farsighted
• -hed = a suffix that turns an adjective into an abstract noun, similar to English -ness
• -en = the definite ending

So roughly:

• langsynet = farsighted
• langsynethed = farsightedness
• langsynetheden = the farsightedness

This pattern is very common in Danish:

• sund = healthy
• sundhed = health
• sundheden = the health

Not normally in standard Danish.

In Danish, singular countable nouns like optiker usually need some kind of determiner or definite marking:

• en optiker = an optician
• optikeren = the optician

So Optiker forklarer ... sounds incomplete in ordinary prose.

For langsynethed, the bare form without the definite ending is possible in some contexts, but it changes the nuance:

• forklarer langsynethed can sound more general, like explaining the concept
• forklarer langsynetheden sounds like a specific instance or the condition being discussed

So the original sentence is the more natural complete sentence here.

A rough guide is:

optikeren forklarer langsynetheden
op-TI-ke-ren for-KLAR-er lang-SY-ne-the-den

A few pronunciation notes:

• r in Danish is softer than in English.
• Final syllables are often less clearly pronounced than English speakers expect.
• y in langsynetheden is a front rounded vowel, which many English speakers find difficult.
• d in endings like -heden is often softer than an English d.

A more approximate learner-friendly version might be:

ob-TI-ger-en for-KLAH-er lang-SY-ne-heh-den

But for accurate pronunciation, it is best to hear it from native audio.

Yes, gender matters in Danish because it affects the indefinite article and sometimes other grammar.

• en optiker
• en langsynethed

Both are common gender nouns, which is why their definite singular forms end in -en:

• optikeren
• langsynetheden

If a noun were neuter, you would usually see -et instead in the definite singular:

• et hus = a house
• huset = the house

So the -en ending here tells you these nouns are common gender.