Mi amiga sonrió en cuanto oyó el primer aplauso.

Because the verb is sonreír and its preterite form is irregular in spelling and stress.

For él/ella/usted, the preterite is sonrió = he/she smiled.

A quick pattern:

• yo sonreí
• tú sonreíste
• él/ella sonrió
• nosotros sonreímos
• vosotros sonreísteis
• ellos sonrieron

Notice that verbs like sonreír and oír often change slightly in the preterite to keep natural pronunciation:

• sonrió
• oyó

So sonreó is not a correct form.

Oír means to hear, while escuchar means to listen.

In this sentence, the idea is that she reacted when she heard the first clap, not necessarily that she was actively listening for it. So oyó is the most natural choice.

• oír = to hear
• escuchar = to listen

You could sometimes say escuchó in a similar context, but it would suggest a bit more active attention.

En cuanto means as soon as or the moment that.

So:

• Mi amiga sonrió en cuanto oyó el primer aplauso
• My friend smiled as soon as she heard the first clap/applause

It introduces a time clause: one action happened immediately after another.

Other common time expressions with a similar meaning are:

• apenas
• tan pronto como
• nada más

But en cuanto is very common and natural.

Because both actions are seen as completed events in the past.

• oyó = she heard
• sonrió = she smiled

The sentence tells a sequence of completed actions:

1. She heard the first clap.
2. She smiled.

In Spanish, the preterite is commonly used for this kind of finished past event.

If the sentence were describing background or an ongoing situation, the imperfect might appear instead, but here the focus is on specific completed moments.

El primer aplauso means the first clap or the first burst/sound of applause.

Spanish often uses aplauso in the singular to refer to a clap or an instance of applause. Here, the sentence focuses on the very first sound that triggered her reaction.

So the meaning is something like:

• she smiled at the first clap
• she smiled as soon as the applause began

If you said los primeros aplausos, that would suggest the first several claps or the opening moments of the applause as a plural idea.

Yes. Amiga is feminine, so mi amiga means my female friend.

Compare:

• mi amigo = my male friend
• mi amiga = my female friend

The possessive mi does not change for masculine or feminine singular nouns:

• mi libro
• mi casa
• mi amiga

So the gender is shown by amiga, not by mi.

The accent marks show where the stress falls and help distinguish the correct verb forms.

• oyó
• sonrió

Without the accent, the pronunciation and sometimes the form would be unclear or incorrect.

These accents are especially common in certain preterite forms of verbs ending in -oír, -eír, or -reír, where vowels come together and Spanish marks the stressed syllable clearly.

For a learner, the key thing is simply to memorize these forms with the accent included:

• oyó
• sonrió

Normally, no. The standard verb here is sonreír, so Mi amiga sonrió is the natural and correct choice.

In standard Spanish, sonreír is usually not used as se sonrió in this context. You simply say:

• sonrió = she smiled

So:

• Mi amiga sonrió... is correct
• Mi amiga se sonrió... is not the normal choice here

Yes, Spanish word order is flexible, but the original order is very natural.

The sentence structure is:

• Mi amiga = subject
• sonrió = main verb
• en cuanto oyó el primer aplauso = time clause

You could also say:

• En cuanto oyó el primer aplauso, mi amiga sonrió

This means the same thing, but it emphasizes the timing a little more.

Both are correct. The original version sounds smooth and neutral.

Yes, oyó is completely normal in Spain.

In everyday speech in Spain, you will hear both oír and escuchar, but they are not exactly the same:

• oír = to hear
• escuchar = to listen

So in this sentence, oyó is very natural in Peninsular Spanish. A speaker from Spain would understand it immediately and likely use it themselves in the right context.