A mi hermana le gusta el cebollino en la tortilla, pero yo prefiero un té de menta después de cenar.

Because gustar works differently from to like in English.

In Spanish, gustar literally works more like to be pleasing to someone. So:

• A mi hermana = to my sister
• le = to her
• gusta = is pleasing

So A mi hermana le gusta el cebollino is literally closer to:

• Chives are pleasing to my sister

That is why Spanish uses an indirect object pronoun like le and often also includes A mi hermana for clarity or emphasis.

Yes, they point to the same person, but this is normal in Spanish.

• le is the required indirect object pronoun
• A mi hermana clarifies or emphasizes who that pronoun refers to

This kind of doubling is very common with gustar and similar verbs:

• A Juan le encanta el fútbol
• A mis padres les gusta viajar

In English it can feel repetitive, but in Spanish it sounds natural.

Because the verb agrees with the thing being liked, not with the person who likes it.

Here, the subject is el cebollino which is singular, so we use:

• gusta

Compare:

• Le gusta el cebollino = He/she likes chives
• Le gustan las cebollas = He/she likes onions

So the key question is: what is doing the pleasing? In this sentence, it is el cebollino, singular.

Spanish often uses the definite article more than English does, especially with general nouns.

So:

• el cebollino = chives / the chives

In English, we often drop the article in general statements, but Spanish usually keeps it:

• Me gusta el café = I like coffee
• No me gusta la leche = I don’t like milk

So el cebollino sounds natural in Spanish even if English would often just say chives.

Here en la tortilla means in the omelette or in the Spanish omelette/tortilla.

In Spain, tortilla usually means an omelette, especially tortilla española. It does not usually mean the flatbread that English speakers may think of from Mexican food.

So in this sentence:

• el cebollino en la tortilla = chives in the omelette

Yes, it could just be pero prefiero.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is:

• prefiero already means I prefer

But yo is added here for contrast:

• A mi hermana le gusta...
• pero yo prefiero...

So yo emphasizes the difference between my sister and me.

Because preferir means to prefer, which is a different idea from gustar.

The sentence is contrasting two things:

• what the sister likes
• what the speaker prefers

So:

• A mi hermana le gusta... = My sister likes...
• yo prefiero... = I prefer...

Also, preferir works more like a normal English verb:

• Yo prefiero un té
• Ella prefiere café

Unlike gustar, the subject is the person doing the preferring.

Because preferir is a stem-changing verb.

In the present tense, the e in the stem changes to ie in most forms:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

So prefiero is the correct yo form.

De menta means mint or more literally of mint.

So:

• un té de menta = a mint tea

This is a very common Spanish structure for describing flavors, ingredients, or types:

• té de limón = lemon tea
• helado de vainilla = vanilla ice cream
• zumo de naranja = orange juice

Spanish often uses an article where English may not.

So prefiero un té de menta sounds natural and means:

• I prefer a mint tea
• or simply I prefer mint tea, depending on context

Using un makes it sound like the speaker wants a cup of mint tea, especially in this situation after dinner.

Después de cenar means after dinner or more literally after eating dinner / after having dinner.

In Spanish, after many prepositions, you use an infinitive when the subject stays general or is understood:

• antes de salir = before leaving
• después de comer = after eating
• sin hablar = without speaking

So:

• después de cenar = after having dinner

This is very natural Spanish.

Yes. The exact English translation depends on context and style.

• cenar = to have dinner / to eat dinner / to have supper

So después de cenar can be rendered as:

• after dinner
• after having dinner
• after eating dinner
• after supper

All are possible, depending on how natural you want the English to sound.

It has two main parts joined by pero:

1. A mi hermana le gusta el cebollino en la tortilla

   • literally: To my sister, chives in the omelette are pleasing

2. pero yo prefiero un té de menta después de cenar

   • but I prefer a mint tea after dinner

So the pattern is:

• [Person] + le gusta + [thing liked]
• pero + yo prefiero + [thing preferred] + [time expression]

It is a nice example of how Spanish can use two different verb patterns in the same sentence:

• gustar-type structure
• regular subject-verb structure with preferir

Yes, especially because of tortilla.

In Spain, tortilla normally means omelette, often the famous potato omelette. In many other Spanish-speaking contexts, especially influenced by Mexican Spanish, English speakers may expect tortilla to mean a flatbread, but that is not the usual meaning in Spain.

The rest of the sentence is standard Spanish that would be understood widely.