Mi prima toca la guitarra y el piano desde niña, pero todavía se pone nerviosa en los conciertos.

Toca is the 3rd person singular present tense of tocar.

• tocar = to play a musical instrument
• mi prima = my cousin (female), so the verb must match she
• That gives us: mi prima toca = my cousin plays

The present tense is used here for a habitual / ongoing ability or activity:

• Mi prima toca la guitarra = My cousin plays the guitar

It does not mean she is playing right this second. It means this is something she does / knows how to do.

After tocar, Spanish normally uses the definite article with musical instruments:

• tocar la guitarra
• tocar el piano
• tocar el violín

This is one of the most common differences from English. English usually says:

• play guitar
• play piano

But Spanish usually says:

• play the guitar
• play the piano

So toca la guitarra y el piano is completely natural.

Both are possible, but they work slightly differently.

• desde niña = since childhood / since she was a girl
• desde que era niña = since she was a girl

In desde niña, Spanish uses a very compact expression:

• desde
  • noun/adjective phrase

It is a shorter way to express an idea that in English often needs a full clause.

Compare:

• Toca el piano desde niña.
• Toca el piano desde que era niña.

Both are correct.
Desde niña is simply more concise.

Because niña here is being used in a general descriptive way, almost like as a child or since childhood, not to refer to one specific girl.

So:

• desde niña = since childhood / since she was little

If you added an article, it would sound different and much less natural in this sentence.

This kind of article omission is common in fixed expressions like:

• de niño / de niña = as a boy / as a girl
• desde pequeño / desde pequeña = since childhood / since being little

Because ponerse nervioso/a means to become / to get nervous, while estar nervioso/a means to be nervous.

So the difference is:

• se pone nerviosa = she gets nervous
• está nerviosa = she is nervous

In this sentence, the idea is that when concerts happen, she becomes nervous, so ponerse is the better choice.

Examples:

• Antes del examen, me pongo nervioso. = I get nervous before the exam.
• Ahora estoy nervioso. = I am nervous now.

Here, se is part of the pronominal verb ponerse.

• poner by itself usually means to put
• ponerse can mean to become in many contexts, especially for changes of state

So:

• pone = puts
• se pone nerviosa = she gets nervous

You should learn ponerse + adjective as a very common pattern:

• ponerse triste = to get sad
• ponerse contento = to become happy
• ponerse enfermo = to get sick
• ponerse nervioso = to get nervous

Because the adjective must agree with mi prima, who is female.

• masculine singular: nervioso
• feminine singular: nerviosa

So:

• Mi primo se pone nervioso.
• Mi prima se pone nerviosa.

This is standard adjective agreement in Spanish.

Todavía here means still.

So:

• pero todavía se pone nerviosa = but she still gets nervous

It emphasizes that this is true even now, despite the fact that she has been playing for a long time.

Its position is very natural before the verb phrase:

• todavía se pone nerviosa

You could also hear aún in similar contexts:

• pero aún se pone nerviosa

Both are common, though todavía is very frequent in everyday speech.

Because Spanish often uses the present tense for an action or situation that:

1. started in the past, and
2. continues into the present

So:

• toca ... desde niña literally uses the present tense
• in English, we often prefer has played ... since childhood

This is a very important pattern:

• Vivo aquí desde 2018. = I have lived here since 2018.
• Estudia español desde pequeña. = She has studied Spanish since she was little.

So the Spanish present tense is perfectly normal here.

Because en is used to mean in / at the context or setting where something happens.

• en los conciertos = at concerts / in concerts

The sentence is saying that she gets nervous during concerts or when she is at concerts performing.

By contrast, a los conciertos would usually suggest movement toward concerts:

• Va a los conciertos = She goes to concerts

So here, en is the correct preposition.

Spanish often uses the definite article to talk about things in a general, repeated, or typical context.

So:

• en los conciertos = at concerts / in concerts generally

It does not necessarily mean some specific concerts already mentioned. It can refer to concerts as the kind of situation where this happens.

Spanish uses definite articles more often than English in these general statements.

In standard Spanish, especially in careful usage, toca la guitarra y el piano is the best choice.

Without the articles:

• toca guitarra y piano

this may sound incomplete or less standard to many speakers, especially in Spain.

So for a learner, the safest rule is:

• after tocar
  • instrument, use the definite article:
    • tocar la guitarra
    • tocar el piano
    • tocar la batería

Because Spanish, like English, can omit the repeated verb when the same verb applies to both items.

So:

• toca la guitarra y el piano

means:

• plays the guitar and the piano

You could repeat the verb:

• Mi prima toca la guitarra y toca el piano

but that sounds more repetitive unless you want emphasis.

So the version in the sentence is the most natural one.