Min søster mener også, at kvarteret er roligere om aftenen.

In Danish, when you use a possessive like min, din, hans, vores, etc., you normally do not add the definite ending to the noun.

So:

• min søster = my sister
• min bror = my brother
• mit hus = my house

Not:

• min søsteren
• mit huset

The definite ending -en / -et is used when the noun means the ..., but not when it already has a possessive in front of it.

Because søster is a common gender noun in Danish.

Danish has two grammatical genders:

• common gender → takes en in the indefinite article and min
• neuter → takes et in the indefinite article and mit

So:

• en søster → min søster
• et hus → mit hus

Since søster is a common gender noun, min is the correct form.

Mener is the present tense of at mene, which often means to think, to mean, or to be of the opinion that.

In this sentence, it means something like:

• thinks
• is of the opinion
• believes

A learner may compare it with other Danish verbs for think:

• at mene = to hold an opinion
• at tro = to believe / think something is probably true
• at synes = to feel / think, often more personal or subjective

So min søster mener også ... sounds like my sister also thinks / is also of the opinion that ...

In a main clause, Danish usually follows verb-second word order. That means the finite verb tends to come early, and adverbs like også often come after it.

Here the structure is:

• Min søster = subject
• mener = finite verb
• også = adverb
• at ... = subordinate clause

So:

• Min søster mener også, at ...

This placement is very natural in Danish.

If you moved også, the emphasis could change. For example:

• Også min søster mener, at ... = My sister too thinks that...
• Min søster mener også, at ... = My sister also thinks that...

Because at kvarteret er roligere om aftenen is a subordinate clause introduced by at.

In Danish writing, many people place a comma before subordinate clauses:

• Min søster mener også, at kvarteret er roligere om aftenen.

You may also see:

• Min søster mener også at kvarteret er roligere om aftenen.

Both can occur, depending on the comma system being used. So the comma is very common, but whether it is strictly required depends on the punctuation style.

Because after at, you are inside a subordinate clause, and subordinate clauses in Danish do not use normal verb-second word order.

In a main clause, Danish usually has:

• Subject + verb + ...

or, if something else comes first:

• Then the verb still comes second

But in a subordinate clause, the order is more straightforward:

• at + subject + verb + ...

So:

• at kvarteret er roligere om aftenen

not

• at er kvarteret roligere om aftenen

This is one of the most important word-order differences Danish learners need to notice.

Because kvarteret is the definite form: the neighborhood / the district.

Danish usually adds the definite article as an ending on the noun:

• et kvarter = a neighborhood / a district
• kvarteret = the neighborhood / the district

Since kvarter is a neuter noun, its definite ending is -et.

So the sentence is talking about the neighborhood, not just a neighborhood.

Roligere is the comparative form of rolig.

• rolig = calm / quiet
• roligere = calmer / quieter
• roligst = calmest / quietest

Danish often forms the comparative by adding -ere to the adjective.

So:

• kvarteret er roligt = the neighborhood is quiet
• kvarteret er roligere = the neighborhood is quieter

Here rolig is being used in the sense of quiet/calm, which is very natural for places, streets, neighborhoods, evenings, and so on.

Om aftenen is a very common Danish time expression meaning in the evening / in the evenings / during the evening, depending on context.

Danish often uses:

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

So om + definite form is normal here.

I aftenen is not the usual way to say in the evening in this general sense. If you mean this evening, Danish usually says:

• i aften = this evening / tonight

So compare:

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

It can, but not here.

Kvarter has several meanings in Danish, including:

• quarter as in a quarter of an hour
• district / neighborhood / area

In this sentence, it clearly means the neighborhood or the area.

This is similar to how one word in English can have different meanings depending on context.

A rough guide:

• søster: the ø is a front rounded vowel that English does not really have. It is one of the sounds English speakers usually need to practice separately.
• også: often pronounced roughly like oh-so, but with Danish vowel quality and smoother speech rhythm.
• aftenen: often sounds more compressed in natural speech than the spelling suggests. The written endings in Danish are frequently pronounced more weakly than English learners expect.

A few useful points:

• ø is not the same as English o
• å is more like an open aw/o sound
• spoken Danish often reduces unstressed syllables, especially endings like -en

So it is a good idea to listen to native audio for the whole sentence, because Danish pronunciation is often less transparent than the spelling suggests.