Biblioteket er roligt om aftenen.

Because -et is the definite article added to the end of many Danish neuter nouns.

• et bibliotek = a library
• biblioteket = the library

Unlike English, Danish usually puts the at the end of the noun as a suffix instead of using a separate word in front.

In Danish, every noun has a grammatical gender, and bibliotek is a neuter noun, so it takes et.

• et bibliotek = a library
• biblioteket = the library

This is something you generally have to learn with the noun. A good habit is to memorize nouns together with their article:

• et bibliotek
• en bog
• et hus

Because roligt agrees with the subject biblioteket, which is a neuter singular noun.

The adjective rolig changes depending on grammar:

• rolig with en-words: en rolig by
• roligt with et-words: et roligt bibliotek
• rolige in plural or definite contexts: rolige børn, det rolige bibliotek

In Biblioteket er roligt, the subject is biblioteket, so the adjective takes the -t form: roligt.

Here it is best understood as an adjective describing the subject biblioteket.

The structure is:

• Biblioteket = subject
• er = is
• roligt = adjective linked to the subject

So the sentence means that the library is quiet/calm.

It may look like an adverb because of the -t ending, but in this sentence it agrees with the neuter noun and functions as a subject complement.

Er is the present tense of at være = to be.

So:

• jeg er = I am
• du er = you are
• biblioteket er = the library is

In this sentence, er links the subject biblioteket with its description roligt.

Om aftenen means in the evening or during the evening.

• om is often used for recurring or general times
• aftenen is the evening

So the phrase gives a time setting: the library is quiet in the evening.

This often sounds like a general statement rather than one specific evening.

In this expression, Danish commonly uses the definite form:

• om morgenen = in the morning
• om eftermiddagen = in the afternoon
• om aftenen = in the evening
• om natten = at night / in the night

So even though English usually says in the evening with a separate word, Danish expresses this with om + definite noun.

Usually it suggests a general or habitual time, not one specific evening.

So Biblioteket er roligt om aftenen normally means something like:

• The library is quiet in the evening
• The library tends to be quiet in the evenings

If you wanted to refer to one specific evening, Danish would often use a more specific time expression.

This is the normal Danish word order for a simple statement:

subject + verb + complement + time expression

So:

• Biblioteket = subject
• er = verb
• roligt = description
• om aftenen = time phrase

This is very similar to standard English word order: The library is quiet in the evening.

Yes. Rolig often covers both ideas depending on context.

It can describe:

• a place that is quiet
• a person who is calm
• a situation that is peaceful

In Biblioteket er roligt, quiet is usually the most natural English translation, because we are talking about a library.

A rough guide is:

bee-blee-oh-TEH-guhd air ROH-leet om AF-teh-nen

A few useful notes:

• biblioteket is often pronounced more smoothly than it looks
• the d in roligt is not pronounced like a strong English d
• er is often said very lightly
• Danish pronunciation is often softer and more reduced than spelling suggests

If you are learning pronunciation, it helps to listen to native audio because Danish spelling and pronunciation often differ quite a bit.

Yes, you could, but the meaning changes slightly.

• roligt = quiet, calm, peaceful
• stille = quiet, silent, still

For a library, both can work, but roligt can suggest a calm atmosphere, while stille focuses more on lack of noise.

So:

• Biblioteket er roligt om aftenen = the library feels calm/quiet in the evening
• Biblioteket er stille om aftenen = the library is silent/very quiet in the evening

Both are possible, but roligt sounds a bit broader and softer.