Antes de volver al hotel, pasamos por un mercadillo lleno de libros viejos y postales.

Because antes de is the normal pattern in Spanish when it is followed by an infinitive.

• antes de + infinitive = before doing something
• Antes de volver al hotel... = Before returning to the hotel...

You can compare:

• Antes de comer, me lavo las manos. = Before eating, I wash my hands.
• Antes de salir, apaga la luz. = Before leaving, turn off the light.

Without de, it sounds incomplete here.

It is in the infinitive because after antes de, Spanish uses the infinitive when the subject is not being stated again separately.

So:

• Antes de volver al hotel... = before returning to the hotel

This works like English before returning or before going back.

If Spanish wanted to use a full conjugated verb, the structure would be different, for example with antes de que:

• Antes de que volviéramos al hotel...

That version is more explicit and usually introduces a new clause.

Al is a contraction of a + el.

• a = to
• el = the
• a + el = al

So:

• volver al hotel = to return to the hotel

This contraction is required in Spanish whenever a is followed by the masculine singular article el.

Compare:

• Voy al museo. = I’m going to the museum.
• Subimos al tren. = We got on the train.

The main exception is when el is part of a proper name:

• Voy a El Escorial.

Here, pasar por means something like to go by, to stop by, or to pass through/by a place, depending on context.

In this sentence, pasamos por un mercadillo suggests that we went by / stopped by a street market on the way somewhere.

This is a common expression:

• Pasamos por tu casa. = We stopped by your house / went by your house.
• Pasé por la tienda. = I stopped by the shop.

So pasar by itself often means to pass, but pasar por is a very common combination with places.

Pasamos is the preterite, which presents the action as a completed event.

• pasamos = we went by / we stopped by

That fits a sentence describing a specific event in a story: first this happened, then something else happened.

If it were pasábamos, that would be the imperfect, which usually gives background, repeated action, or an ongoing scene:

• Antes de volver al hotel, pasábamos por ese mercadillo todos los días. = Before going back to the hotel, we used to go through that market every day.

So in your sentence, pasamos sounds like one completed action.

Not exactly. Mercadillo is the diminutive form of mercado.

• mercado = market
• mercadillo = little market, street market, flea market, small market

The ending -illo / -illa often adds the idea of something smaller, more informal, or sometimes more affectionate.

In Spain, mercadillo often refers to an open-air market or street market, sometimes one held on specific days.

So it is not always literally just a small market; it can also suggest a certain type of market.

Because lleno agrees with mercadillo, which is:

• singular
• masculine

So:

• un mercadillo lleno de...

If the noun changed, the adjective would change too:

• una tienda llena de libros
• unos puestos llenos de gente
• unas cajas llenas de postales

This is standard adjective agreement in Spanish.

Yes, lleno normally goes with de when you say what something is full of.

• lleno de = full of

Examples:

• lleno de gente = full of people
• llena de flores = full of flowers
• llenos de recuerdos = full of memories

So in your sentence:

• un mercadillo lleno de libros viejos y postales = a market full of old books and postcards

That de is part of the normal pattern.

In Spanish, adjectives usually come after the noun, especially when they describe a factual quality.

• libros viejos = old books

Here, viejos is just describing the books in a straightforward way: they are old.

If you put the adjective before the noun, it can sound more literary, subjective, or can slightly change the nuance:

• viejos libros

That might sound more expressive or poetic, depending on context.

For everyday description, libros viejos is the most natural choice.

Because Spanish often leaves out the article when talking about plural nouns in a general, non-specific way, especially in lists after expressions like lleno de.

So:

• lleno de libros viejos y postales

means the market was full of old books and postcards in general, not some specific previously mentioned postcards.

You could include an article in some contexts, but it would usually make the noun phrase more specific:

• lleno de los libros y las postales que compramos
  = full of the books and postcards that we bought

In your sentence, no article sounds natural.

No. Spanish allows some flexibility in word order.

Your sentence begins with that phrase because it sets the scene nicely:

• Antes de volver al hotel, pasamos por un mercadillo...

But you could also say:

• Pasamos por un mercadillo lleno de libros viejos y postales antes de volver al hotel.

Both are grammatical. The version with antes de volver al hotel at the beginning feels a bit more narrative and gives the time frame first.

Yes, normally it is understood to be the same subject.

• pasamos = we went by / we stopped by
• antes de volver = before returning

So the natural reading is:

• Before we returned to the hotel, we stopped by...

Spanish often uses the infinitive this way when the subject is the same as in the main clause.

If the subject were different, Spanish would often use a different structure, such as antes de que + subjunctive.

Example:

• Antes de que ellos volvieran al hotel, pasamos por el mercadillo. = Before they returned to the hotel, we went by the market.