Mi hermana prefirió una ración de croquetas y dijo que el aperitivo estaba muy bueno.

Because preferir is a stem-changing verb. In the preterite, the e in the stem changes to i in the third-person forms:

• yo preferí
• tú preferiste
• él/ella prefirió
• nosotros preferimos
• vosotros preferisteis
• ellos/ellas prefirieron

So mi hermana prefirió means my sister preferred / chose.

Prefirió is the preterite, which presents the action as completed: she made a choice at that moment.

• prefirió = she preferred / she chose
• prefería = she used to prefer / she was preferring / she generally preferred

In this sentence, it sounds like a specific event, probably during one meal or one occasion, so prefirió fits better.

In Spain, una ración usually means a serving or a portion, especially in a bar or restaurant.

So:

• una ración de croquetas = a portion of croquettes

In Spain, food is often ordered as:

• una tapa
• media ración
• una ración

A ración is usually bigger than a tapa.

Croquetas are a very common Spanish food: small breaded and fried rolls, usually filled with a creamy mixture such as ham, chicken, cod, or mushrooms.

So although croquetas resembles English croquettes, Spanish croquetas are a very specific and very popular tapas dish.

With decir, standard Spanish normally uses que directly:

• dijo que... = she said that...

So:

• dijo que el aperitivo estaba muy bueno

is correct.

Adding de here would be incorrect in standard Spanish. This is a common learner issue because some verbs do require de que, but decir does not.

For food, Spanish often uses estar bueno to mean to taste good or to be delicious.

• El aperitivo estaba muy bueno = the appetizer tasted very good / was really good

Using ser bueno would sound more like talking about its general quality, character, or suitability, not so much the immediate eating experience.

So with food, estar bueno is very natural.

Because bueno agrees with el aperitivo, which is masculine singular.

• el aperitivo → bueno
• la croqueta → buena
• las croquetas → buenas

Adjectives in Spanish usually agree in gender and number with the noun they describe.

In Spain, aperitivo can mean an appetizer, a pre-meal snack, or even the social moment of having a drink and something small to eat before lunch.

In this sentence, it most likely means the appetizer / starter / snack they were having.

Here, estaba describes a state or quality in the past: the appetizer was very good.

The imperfect is very common for background description, opinions, and ongoing states in the past.

• estaba muy bueno = it was very good
• estuvo muy bueno can also exist, but it sounds more like viewing the whole experience as a completed event

In reported speech after dijo que, estaba is especially natural when reporting what something was like.

Yes, very natural.

For food in Spain, people often say:

• Está muy bueno
• Está buenísimo
• Está rico
• Está riquísimo

So el aperitivo estaba muy bueno sounds completely normal and idiomatic.

Spanish usually does not use an article before a possessive adjective like mi, tu, su, etc.

So you say:

• mi hermana
• tu coche
• su casa

not:

• la mi hermana

That structure is not standard modern Spanish.

The personal a is used before a specific human direct object.

Examples:

• Veo a mi hermana
• Conozco a Juan

But in your sentence:

• mi hermana is the subject, not the object
• una ración de croquetas is a thing, not a person
• el aperitivo is also a thing

So no personal a is needed.

Roughly: preh-feh-ree-OH

The written accent on ó shows that the stress falls on the last syllable:

• pre-fi-RIÓ

That accent is important because without it, the stress pattern would be different and the form would be incorrect.

Yes. In context, preferir often means not just having a preference in general, but actually choosing one option over another.

So:

• Mi hermana prefirió una ración de croquetas

can suggest:

• My sister preferred a portion of croquettes
• My sister opted for a portion of croquettes
• My sister chose a portion of croquettes

All are reasonable depending on context.