A mi tía no le gustan los mejillones, pero mi prima pide calamares cuando vamos al bar del puerto.

Because los mejillones is plural, and with gustar the verb agrees with the thing being liked or disliked.

• A mi tía no le gusta el mejillón = if it were singular
• A mi tía no le gustan los mejillones = plural

So the grammatical subject is los mejillones, not mi tía.

Yes, they both refer to the aunt, but this is normal in Spanish. It is called indirect object doubling.

• le is the indirect object pronoun
• a mi tía specifies who le refers to

With gustar, the pronoun is especially important and is usually kept. The longer phrase often adds clarity or emphasis.

So:

• Le gustan los mejillones = She likes mussels / Mussels please her
• A mi tía le gustan los mejillones = makes it clear that her means my aunt

A useful way is to think of gustar as meaning to be pleasing to, not as a direct equivalent of English to like.

So:

• A mi tía no le gustan los mejillones
• more literally: Mussels are not pleasing to my aunt

That is why:

• the person is introduced with a and represented by le
• the verb agrees with los mejillones

This is one of the most important patterns to get used to in Spanish.

In Spanish, no normally goes before the verb, and object pronouns such as le come before the conjugated verb too.

So the order is:

• no + le + gustan

Not:

• le no gustan
• gustan no

This same pattern appears in many sentences:

• No me gusta
• No te entiendo
• No les interesa

Because the two nouns are being used in different ways.

1. los mejillones

After gustar, Spanish often uses the definite article when talking about things in general.

• Me gusta el café
• No me gustan los perros
• A mi tía no le gustan los mejillones

This does not mean specific mussels. It can mean mussels as a type of food.

2. calamares

After pedir, it is very common to leave the noun without an article when talking about what someone orders.

• Pide calamares
• Pido café
• Pedimos tortilla

Here it means she orders some squid/calamari, or a squid dish.

Yes, but it changes the nuance.

• pide calamares = she orders calamari/squid as a dish, in a general sense
• pide los calamares = more likely a specific dish, portion, or the calamari already identified in the context

In a restaurant or bar, the version without the article is very common when simply saying what someone orders.

In Spain, pedir is the normal verb for ordering food or drink.

So:

• Mi prima pide calamares = My cousin orders calamari

For many learners, ordenar looks tempting because of English order, but in Spain ordenar usually means:

• to put in order
• to organize
• to command

In some Latin American varieties, ordenar can be used for ordering food, but pedir is the safest and most natural choice here, especially for Spain.

Because this sentence talks about a habitual action: something that happens whenever we go there.

• cuando vamos al bar del puerto = when/whenever we go to the bar by the port

Spanish uses the indicative after cuando for repeated or habitual actions.

If you said cuando vayamos, that would sound more like a future or not-yet-real event:

• cuando vayamos al bar del puerto = when we go to the bar by the port / next time we go

So:

• cuando vamos = habitual
• cuando vayamos = future/uncertain

They are contractions.

• a + el = al
• de + el = del

So:

• vamos al bar = we go to the bar
• el bar del puerto = the bar of/by the port

These contractions are required in standard Spanish whenever a or de comes directly before el.

But there is no contraction with la:

• a la playa
• de la ciudad

It means the bar at/by the port or the port bar, depending on context.

Literally, del puerto is of the port, but in natural English we often translate it more smoothly as:

• the bar by the port
• the bar at the port

Also, in Spain, bar often refers to a casual place for drinks, coffee, tapas, sandwiches, and simple meals. It does not necessarily mean a drinking-only bar in the English sense.

Because Spanish often leaves subject pronouns out when they are not needed.

The verbs already show the person:

• pide = she/he orders
• vamos = we go

And in this sentence, the subjects are already clear:

• mi prima tells us who pide
• vamos naturally means we go

Spanish usually adds subject pronouns only for emphasis, contrast, or clarification.

Yes, but mostly for emphasis and flow.

Starting with A mi tía highlights the aunt first, which makes the contrast with pero mi prima feel natural:

• A mi tía no le gustan los mejillones, pero mi prima...

It sets up the idea:

• my aunt dislikes one thing
• but my cousin does something different

Spanish word order is often more flexible than English, but this version sounds very natural and clear.