Mi amiga se puso nerviosa, pero sonrió en cuanto oyó el primer aplauso.

Because ponerse + adjective often means to become / to get a certain way.

So:

• se puso nerviosa = she became/got nervous
• estaba nerviosa = she was nervous

In this sentence, the idea is that her emotional state changed at that moment, so ponerse fits better than estar.

Here, se is part of the verb ponerse. It does not literally mean herself in the English sense.

• poner = to put
• ponerse = to become / to put on / to get into a state, depending on context

In ponerse nerviosa, the whole expression means to get nervous.

So it is best to learn ponerse nervioso/a as a set expression.

Because nerviosa agrees with mi amiga, which is feminine singular.

Spanish adjectives usually agree in gender and number with the noun they describe:

• amigo nervioso
• amiga nerviosa
• amigos nerviosos
• amigas nerviosas

Since amiga is feminine singular, nerviosa must also be feminine singular.

They are in the preterite because the sentence tells a sequence of completed events in the past.

• se puso nerviosa = she got nervous
• sonrió = she smiled
• oyó = she heard

This is typical storytelling Spanish: one event happened, then another, then another.

If the sentence were describing background or an ongoing state, Spanish might use the imperfect instead. But here the actions are viewed as specific and completed.

The accent mark shows the stress and also helps distinguish the correct verb form.

sonrió is the third-person singular preterite of sonreír:

• yo sonreí
• tú sonreíste
• él/ella sonrió

The stress falls on the final syllable: son-ri-Ó.

Without the accent, the pronunciation and spelling would not match the standard form.

Both can often be translated as to hear/listen, but there is a difference:

• oír = to hear
  more about perceiving a sound
• escuchar = to listen
  more about paying attention to a sound

In this sentence, oyó el primer aplauso means she heard the first applause/the first clap. It focuses on the moment the sound reached her.

You could imagine escuchó in some contexts, but oyó is very natural here because it is about the instant she heard it.

En cuanto means as soon as.

So:

• sonrió en cuanto oyó el primer aplauso
  = she smiled as soon as she heard the first applause

It introduces an event that happens immediately after another one.

No. It depends on whether the action is viewed as real/completed or future/expected.

Here it uses the indicative because the whole sentence refers to a completed past event:

• sonrió en cuanto oyó...

But if you are talking about the future, Spanish often uses the subjunctive after en cuanto:

• Sonreirá en cuanto oiga el primer aplauso.
  = She will smile as soon as she hears the first applause.

So:

• past or known fact → usually indicative
• future or pending action → usually subjunctive

Because primero shortens to primer before a masculine singular noun.

So:

• el primer aplauso
• el primer día
• el primer libro

But:

• el primero = the first one
• la primera vez = the first time

This shortening is very common with primero and also with tercero:

• el tercer acto

It can suggest either, depending on context.

• aplauso can mean a clap
• it can also refer to applause, especially as a burst or round of clapping

In this sentence, el primer aplauso most naturally means the first clap / the first burst of applause that she heard from the audience.

So English could translate it in slightly different ways depending on style.

Because pero means but and joins two contrasting parts of the sentence:

• Mi amiga se puso nerviosa
• pero sonrió en cuanto oyó el primer aplauso

The comma helps mark that contrast. This is very normal in Spanish and also common in English before but when joining full clauses.

Yes, but it changes the meaning slightly.

• se puso nerviosa = she got nervous
• estaba nerviosa = she was nervous

So:

• Mi amiga se puso nerviosa... focuses on the moment she became nervous.
• Mi amiga estaba nerviosa... describes her state more generally.

Both are possible, but the original sentence highlights a change of emotion right before she smiled.

In modern standard Spanish, possessives like mi, tu, su, nuestro normally go directly before the noun without an article:

• mi amiga
• tu casa
• su coche

So mi amiga is the normal standard form for my friend.

Forms like la mi amiga are not standard in modern general Spanish.

A simple pronunciation guide is:

Mi amiga se puso nerviosa, pero sonrió en cuanto oyó el primer aplauso.

Approximate pronunciation:

mee ah-MEE-gah seh POO-so ner-BYOH-sah, PEH-ro son-ree-OH en KWAN-to oh-YOH el pree-MEHR ah-PLAU-so

A few useful points:

• ll is not in this sentence, so nothing tricky there
• j is not here either
• rió in sonrió is pronounced smoothly, with the stress on -ó
• oyó is stressed on the last syllable: o-YÓ

In Spain, pronunciation will vary slightly by region, but this guide is close enough for a learner.