Jeg bliver roligere, når jeg lytter til rolig musik.

Bliver means becomes / gets, while er means is / am / are.

So:

• Jeg er rolig = I am calm
• Jeg bliver roligere = I become calmer / I get calmer

In this sentence, the idea is that listening to calm music causes a change of state, so bliver is the natural choice.

Roligere is the comparative form of rolig.

• rolig = calm
• roligere = calmer

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

So:

• Jeg bliver rolig = I become calm
• Jeg bliver roligere = I become calmer

The sentence suggests that the speaker is calmer than before.

Because the two adjectives are doing different jobs.

• roligere describes jeg after bliver. It is a predicate adjective, and it is in the comparative form: calmer.
• rolig describes musik directly. It is an attributive adjective, and it is in the basic form: calm music.

So:

• Jeg bliver roligere = I become calmer
• rolig musik = calm music

They do not need to match each other, because they describe different things.

Because musik here is:

• singular
• indefinite
• usually treated as a common-gender noun in this kind of phrase

With a common-gender singular indefinite noun, the adjective normally stays in its base form:

• en rolig sang = a calm song
• rolig musik = calm music

You would use rolige in other situations, for example:

• de rolige sange = the calm songs
• rolige børn = calm children

So rolig musik is correct.

Because the verb at lytte normally goes with til when you say what someone is listening to.

• lytte til musik = listen to music
• lytte til radioen = listen to the radio
• lytte til dig = listen to you

This is just how the verb works in Danish.

Compare:

• at høre musik = to hear music
• at lytte til musik = to listen to music

Høre is more passive, while lytte til is more active and intentional.

This is a very common question.

• høre = hear
• lytte til = listen to

Examples:

• Jeg hører musik = I hear music / I am listening to music
  (In everyday Danish, this can also mean I’m listening to music.)
• Jeg lytter til musik = I listen to music / I am listening to music
  (This sounds more focused and deliberate.)

So in your sentence, lytter til emphasizes the act of listening.

Because after når, Danish usually keeps normal subordinate-clause word order:

• når jeg lytter til rolig musik

That means:

1. conjunction: når
2. subject: jeg
3. verb: lytter

This is different from main clause inversion, which happens after certain adverbs or when another element comes first in a main clause.

For example:

• Jeg bliver roligere, når jeg lytter til rolig musik.
• Når jeg lytter til rolig musik, bliver jeg roligere.

In the second version, the main clause has inversion after the fronted subordinate clause:

• bliver jeg not
• jeg bliver

Here når means when.

It is used for something that happens regularly, generally, or whenever something else happens.

So:

• Jeg bliver roligere, når jeg lytter til rolig musik. = I become calmer when / whenever I listen to calm music.

Hvis means if and is more conditional:

• Jeg bliver roligere, hvis jeg lytter til rolig musik. = I become calmer if I listen to calm music.

That version is also possible, but it has a slightly different meaning. Når suggests this is a normal pattern; hvis sounds more hypothetical.

Because Danish uses commas before subordinate clauses much more regularly than English does.

In this sentence:

• Jeg bliver roligere, når jeg lytter til rolig musik.

the part starting with når is a subordinate clause, so the comma is standard Danish punctuation.

English often omits a comma in similar sentences, but Danish normally includes it.

Usually, roligere is the natural and standard comparative form.

• rolig → roligere

So Jeg bliver roligere is the best choice.

Mere rolig may occasionally appear in speech for emphasis or style, but learners should stick with roligere.

As a general rule in Danish:

• short, common adjectives often take -ere
• mere is more common with adjectives that do not easily take the ending

Here musik means music in general, not one specific song.

So:

• rolig musik = calm music / soothing music

If you wanted to talk about a specific song, you would usually use a different noun, for example:

• en rolig sang = a calm song

In this sentence, musik works like an uncountable noun, much like English music.

Yes, absolutely. That is a very natural alternative.

Both are correct:

• Jeg bliver roligere, når jeg lytter til rolig musik.
• Når jeg lytter til rolig musik, bliver jeg roligere.

The difference is mostly about focus and style.

• Starting with Jeg bliver roligere puts the main point first.
• Starting with Når jeg lytter til rolig musik emphasizes the condition or situation first.

Remember that when the sentence begins with the subordinate clause, the main clause must use inversion:

• bliver jeg not
• jeg bliver