Después del tráiler, nos sentamos más atrás.

Because del is the contraction of de + el.

• después de = after
• el tráiler = the trailer
• So después de el tráiler becomes después del tráiler

This contraction is required in standard Spanish whenever de is followed by el (except when El is part of a proper name, like El Escorial).

Here tráiler means trailer, usually a film trailer in this kind of sentence.

The accent mark shows the stressed syllable: TRÁI-ler.

A few useful notes:

• In Spain, tráiler is a common borrowed word.
• It is masculine: el tráiler
• Depending on context, tráiler can also mean a large truck/trailer, but in this sentence the meaning is understood from context.

It separates the introductory time phrase from the main clause.

So the structure is:

• Después del tráiler = introductory phrase
• nos sentamos más atrás = main action

The comma is natural and helpful here, especially in writing. With short introductory phrases, Spanish sometimes omits the comma, but using it is very normal.

Because the verb here is sentarse, which means to sit down.

• nos sentamos = we sat down / we sat ourselves
• sentamos without nos would usually come from sentar, which means something like to seat someone/something, or in some contexts to suit someone.

So:

• Nos sentamos = We sat down.
• Sentamos a los niños = We seated the children.

In this sentence, the reflexive pronoun nos is necessary.

It can be either, depending on context.

For sentarse, nos sentamos can mean:

• we sit down (present)
• we sat down (preterite / simple past)

In this sentence, the context Después del tráiler makes the past reading the natural one:

• After the trailer, we sat further back.

Spanish sometimes has forms that are identical in the present and preterite for nosotros.

Because:

• más = more / further
• atrás = back / behind / toward the back

Together, más atrás means further back or more toward the back.

This is very natural when talking about places like:

• seats in a cinema
• seats in a classroom
• positions in a room or vehicle

So nos sentamos más atrás means they moved to a position farther toward the back.

Because atrás and detrás are similar, but not used in exactly the same way.

• atrás often means at the back / back / further back
• detrás de usually means behind something specific

Examples:

• Nos sentamos más atrás. = We sat further back.
• Nos sentamos detrás de ellos. = We sat behind them.

In your sentence, the idea is about moving to a more rear position, not being behind a specific object or person, so más atrás is the best choice.

Because the comparison is implicit.

más atrás already means further back without needing to say exactly than what. English does the same thing.

If you want to make the comparison explicit, then you can add que:

• Nos sentamos más atrás que antes. = We sat further back than before.
• Nos sentamos más atrás que vosotros. = We sat further back than you.

But in the original sentence, the reference point is understood, so que is not needed.

Yes, in some contexts you could, but it would not mean exactly the same thing in tone or viewpoint.

• Nos sentamos más atrás = simple past; tells the action as a completed event in the story
• Nos hemos sentado más atrás = present perfect; often connects the action more closely to the present moment

In Spain, the present perfect is used quite a lot, but the preterite is still very normal for narrating a finished event. In this sentence, nos sentamos sounds like straightforward storytelling.

Sometimes, but the meaning changes slightly.

• más atrás = further back
• al fondo = at the back / in the back

So:

• Nos sentamos más atrás suggests a movement to a position farther back than before or than some other position.
• Nos sentamos al fondo means they sat at the very back area.

Both are possible in a cinema or room, but they are not identical. Más atrás is comparative; al fondo is more like a fixed location.