Quando eu fico nervoso, prefiro ouvir música para ficar calmo.

Because they do different jobs in the sentence.

• fico = I become / I get / I am
  It is the 1st person singular present tense of ficar.

  • Quando eu fico nervoso = When I get nervous

• ficar = the infinitive form, used after para

  • para ficar calmo = to become/stay calm

So:

• eu fico = a conjugated verb
• para ficar = an infinitive phrase meaning in order to get/stay

Ficar is a very flexible verb in Portuguese. Here it means something like:

• to become
• to get
• sometimes to stay/remain, depending on context

In this sentence:

• ficar nervoso = to get nervous
• ficar calmo = to become/stay calm

Portuguese often uses ficar + adjective where English uses get + adjective.

Examples:

• Fiquei triste. = I got sad.
• Ela ficou doente. = She got sick.
• Fique tranquilo. = Stay calm / Be calm.

So yes, ficar can mean to stay, but here get/become is the more natural idea.

Yes, it could.

Portuguese often drops subject pronouns because the verb ending already shows who the subject is.

So both are natural:

• Quando eu fico nervoso...
• Quando fico nervoso...

Including eu can:

• add clarity
• add emphasis
• sound a little more explicit

In everyday Brazilian Portuguese, omitting eu is very common, but keeping it is also completely correct.

Because preferir is usually followed directly by an infinitive when the next action is another verb.

Structure:

• preferir + infinitive

So:

• prefiro ouvir música = I prefer to listen to music
• prefiro estudar à noite = I prefer to study at night

No extra preposition is needed before ouvir here.

Also, many learners compare Portuguese to English prefer to listen, but Portuguese does not need a word corresponding to that to in this structure.

Here para means to / in order to and shows purpose.

So:

• ouvir música para ficar calmo = listen to music to become/stay calm
• more literally: listen to music in order to become calm

This is a very common structure:

• para + infinitive

Examples:

• Estudo para aprender. = I study to learn.
• Corro para ficar em forma. = I run to stay in shape.

Because ouvir música usually means to listen to music in general, not a specific song or piece of music.

• ouvir música = listen to music
• ouvir a música = listen to the song / the music (specific)

So in this sentence, the general idea is meant:

• prefiro ouvir música = I prefer listening to music

If you were referring to something specific, then the article would make sense:

• Prefiro ouvir a música que você mandou. = I prefer listening to the song you sent.

Because they agree with the speaker, who is understood here as masculine.

In Portuguese, adjectives usually agree in gender and number with the person or thing they describe.

If the speaker is male:

• nervoso
• calmo

If the speaker is female:

• nervosa
• calma

So a woman would say:

• Quando eu fico nervosa, prefiro ouvir música para ficar calma.

Often yes, but not always exactly.

In Brazilian Portuguese, nervoso can mean:

• nervous
• tense
• upset
• agitated
• irritated, depending on context

So it is a bit broader than English nervous.

Examples:

• Estou nervoso antes da prova. = I’m nervous before the test.
• Ele ficou nervoso com a notícia. = He got upset/agitated because of the news.

Depending on what you mean, Brazilians might also use words like:

• ansioso / ansiosa = anxious
• estressado / estressada = stressed
• tenso / tensa = tense

Yes. Both can mean to listen/hear, and in many everyday situations either one works.

• ouvir música
• escutar música

In practice:

• ouvir is extremely common
• escutar can sound a little more like listen to, but the distinction is not always strict in real usage

Many learners are taught:

• ouvir = to hear
• escutar = to listen

But actual Brazilian Portuguese often overlaps more than that.

For this sentence, prefiro ouvir música is very natural.

Because quando means when, while se means if, and they express different ideas.

• quando = this happens at times / whenever it happens
• se = maybe it happens, maybe not

So:

• Quando eu fico nervoso, prefiro ouvir música...
  = When(ever) I get nervous, I prefer to listen to music...

This suggests a habitual reaction.

Compare:

• Se eu ficar nervoso, vou ouvir música.
  = If I get nervous, I’ll listen to music.

That sounds more like a specific possible future situation.

The present tense here expresses a habitual or general action.

• Quando eu fico nervoso, prefiro ouvir música... = Whenever I get nervous, I prefer to listen to music...

This is a general truth about the speaker’s behavior.

If you changed it, the meaning would shift:

• Quando eu ficar nervoso... = When I get nervous...
  This can sound more tied to a future event or a more specific situation.

• Quando eu estiver nervoso... = When I am nervous...
  This focuses more on the state than the change into that state.

So the original sentence is natural because it describes a repeated pattern.

Yes, it is natural and correct.

It sounds like standard, clear Brazilian Portuguese. Depending on style, people might also say:

• Quando fico nervoso, gosto de ouvir música para me acalmar.
• Quando fico nervoso, eu ouço música para relaxar.
• Quando fico nervoso, prefiro escutar música para me acalmar.

A useful note:

• para ficar calmo = to become/stay calm
• para me acalmar = to calm myself down / to calm me down

Both are natural, but the original sentence is perfectly good.

It breaks down like this:

• Quando eu fico nervoso
  = time clause: When I get nervous

• prefiro ouvir música
  = main clause: I prefer to listen to music

• para ficar calmo
  = purpose: to become/stay calm

So the full structure is:

• When + I get nervous, + I prefer + infinitive, + in order to + infinitive

This is a very common Portuguese pattern:

1. a time clause
2. a main clause
3. a purpose phrase with para + infinitive

That makes the sentence a very useful model for building your own:

• Quando eu fico cansado, prefiro dormir cedo para acordar melhor.
• Quando fico triste, prefiro sair para caminhar.