Jeg læser ofte et digt højt, når jeg vil slappe af om aftenen.

Læser is the present tense form of læse.

In Danish, the present tense is often used for habitual actions, just like English I read in I often read a poem aloud.

• at læse = to read
• jeg læser = I read / I am reading depending on context

So here jeg læser ofte ... means I often read ... as a regular habit.

In a normal Danish main clause, the finite verb usually comes in second position. Since jeg is first, læser must come next, and adverbs like ofte usually come after that.

So the neutral order is:

• Jeg læser ofte et digt højt

not usually:

• Jeg ofte læser ...

A rough pattern is:

• subject + finite verb + adverb + rest

This is very common in Danish main clauses.

Because digt is a neuter noun in Danish, so it takes et in the singular indefinite form.

• et digt = a poem
• digtet = the poem

Danish nouns are either common gender (en-words) or neuter (et-words), and you usually have to learn the gender with the noun.

Here højt means aloud / out loud.

The adjective is høj = high / loud, but in this expression Danish uses the adverbial form højt.

So:

• høj = adjective
• højt = adverbial form

In the phrase læse højt, the meaning is to read aloud.

This is a very common fixed expression in Danish, so it is best to learn læse højt as a set phrase.

Because Danish often puts this kind of adverbial element near the end of the clause.

So:

• Jeg læser ofte et digt højt

sounds natural and neutral.

The structure is roughly:

• Jeg = subject
• læser = finite verb
• ofte = adverb of frequency
• et digt = object
• højt = adverb describing how the reading is done

Placing højt at the end is the most natural word order here.

Når is used for when in the sense of something that happens regularly, repeatedly, or is expected to happen.

That fits this sentence well, because it describes a repeated situation:

• I often read a poem aloud when I want to relax in the evening

By contrast, hvis means if and is more conditional:

• Hvis jeg vil slappe af ... = If I want to relax ...

So når is the natural choice for a habitual or recurring situation.

Vil can mean will, but very often it means want to or intend to.

Here, når jeg vil slappe af means something like:

• when I want to relax
• when I feel like relaxing
• when I’m looking to unwind

So vil is not mainly future here. It expresses wish/intention.

If you said:

• når jeg slapper af om aftenen

that would mean more like:

• when I relax in the evening
• when I am relaxing in the evening

That is a slightly different idea.

Because slappe af is a phrasal/separable verb meaning to relax.

The full verb is:

• at slappe af

When used with a modal verb like vil, the infinitive stays as:

• vil slappe af

So af belongs to slappe, not to om aftenen.

Compare:

• Jeg slapper af = I relax / I am relaxing
• Jeg vil slappe af = I want to relax

It is best to learn slappe af as one unit.

Om aftenen means in the evening or in the evenings, in a general/habitual sense.

It is used for time expressions about parts of the day in a general way:

• om morgenen = in the morning
• om dagen = during the day / in the daytime
• om aftenen = in the evening

This is different from:

• i aften = tonight

So:

• om aftenen = generally, in the evening
• i aften = this evening / tonight

Yes. You can say:

• Når jeg vil slappe af om aftenen, læser jeg ofte et digt højt.

That is completely natural.

But when the subordinate clause comes first, Danish still keeps the finite verb in second position in the main clause, so you get:

• ..., læser jeg ...

not:

• ..., jeg læser ...

This is a very important Danish word-order pattern.

The comma marks the beginning of the subordinate clause:

• når jeg vil slappe af om aftenen

In Danish, you may see both styles:

• Jeg læser ofte et digt højt, når jeg vil slappe af om aftenen.
• Jeg læser ofte et digt højt når jeg vil slappe af om aftenen.

Both can occur depending on the comma system being used. Many writers include the comma before subordinate clauses, and that is what you see here.

So the comma is helping show that the second part is a dependent clause introduced by når.