El camarero dejó los bocadillos sobre la barra y luego nos trajo otra ración de aceitunas.

Because the sentence describes completed actions in the past:

• dejó = he left / put down
• trajo = he brought

The waiter did two specific things, one after the other, so Spanish uses the preterite.

If the sentence were describing an ongoing situation or background, Spanish might use the imperfect instead, but here the actions are clearly seen as finished events.

Both can sometimes be translated as put, but they are not exactly the same.

• dejar often means to leave something somewhere
• poner means to place / put something somewhere

So dejó los bocadillos sobre la barra suggests the waiter left the sandwiches on the counter. It feels very natural in this context.

Nos means to us. It is an indirect object pronoun.

So:

• trajo = he brought
• nos trajo = he brought us

In Spanish, this pronoun is very commonly used even when English would just say he brought us... without thinking about grammar.

You could also say nosotros for emphasis, but normally that is not needed:

• Luego nos trajo otra ración... = normal
• Luego nos trajo a nosotros otra ración... = emphatic, unusual unless contrasting with someone else

Because otro / otra usually already includes the idea of an additional / another, so Spanish normally does not use un/una before it.

So:

• otra ración = another portion
• not una otra ración

This is the normal pattern in Spanish:

• otro café
• otra mesa
• otro día

In Spain, una ración usually means a portion / serving of food, especially in bars or casual restaurants.

So otra ración de aceitunas means:

• another serving of olives
• another portion of olives

This is very common in Peninsular Spanish restaurant and bar language.

Because Spanish often uses de after words for quantity, measure, or serving.

So:

• una ración de aceitunas = a portion of olives
• un vaso de agua = a glass of water
• una taza de café = a cup of coffee

The de links the serving/container/amount to the thing being served.

In Spain, bocadillo usually means a sandwich made with bread like a baguette or roll, not the sliced-bread kind.

That is different from:

• sándwich = usually a sandwich made with sliced bread
• bocadillo = a sandwich in a crustier bread roll or baguette-style bread

So for a learner of Spanish from Spain, that distinction is useful.

Spanish uses articles more often than English.

Here:

• los bocadillos = the sandwiches
• la barra = the counter / bar

The sentence refers to specific things in the situation, so the definite article sounds natural.

Spanish often includes the article where English might sometimes sound fine without it, especially when the objects are understood from context.

Here la barra means the bar counter or the counter.

In a café or bar in Spain, la barra is the long counter where drinks and food may be served or placed.

So sobre la barra means on the counter.

Sobre means on top of, which fits the idea of physically placing the sandwiches on the counter surface.

• sobre la barra = on the counter

If you said en la barra, it could sometimes sound more like at the bar/counter area rather than specifically on top of it.

So sobre is more precise here.

Because traer has an irregular preterite stem.

The preterite forms are:

• traje
• trajiste
• trajo
• trajimos
• trajisteis
• trajeron

So it does not follow the regular pattern of verbs like comer or vivir.

A useful thing to notice is that it behaves similarly to decir in the preterite:

• decir → dije, dijiste, dijo...
• traer → traje, trajiste, trajo...

Luego means then / afterwards / later.

It connects the two actions in sequence:

1. The waiter left the sandwiches on the counter.
2. Then he brought us another serving of olives.

It is a very common word for narrating events in order.

Yes. Spanish word order is flexible, though this version is very natural.

For example, you could also say:

• Luego el camarero nos trajo otra ración de aceitunas.
• El camarero dejó los bocadillos sobre la barra y nos trajo luego otra ración de aceitunas.

But the original sentence sounds smooth and neutral. The placement of luego helps show the sequence clearly.

Because the subject is the same in both actions, and Spanish often avoids repeating it unnecessarily.

So:

• El camarero dejó... y luego nos trajo...

naturally means that the same waiter did both things.

Repeating el camarero would be possible, but it would usually sound less elegant unless you wanted special emphasis.

Grammatically, here it means to us, because it is the indirect object of traer.

But in natural English translation, we usually just say:

• he brought us another serving of olives

rather than

• he brought another serving of olives to us

So the Spanish pronoun is indirect-object nos, even though English often expresses the idea more simply.