Si el tráiler no me gusta, elegiré otra película de la cartelera y me sentaré más atrás.

Because si without an accent means if.

• si = if
• sí = yes

So in this sentence, Si el tráiler no me gusta... means If I don’t like the trailer..., not Yes....

Because gustar works differently from to like in English.

Spanish structures it more like:

• The trailer is pleasing to me → El tráiler me gusta

So:

• me = to me
• gusta = is pleasing

That is why me gusta is correct, not yo gusto el tráiler.

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

Here, the thing being liked is el tráiler, which is singular, so Spanish uses gusta.

Compare:

• Me gusta el tráiler = I like the trailer
• Me gustan los tráileres = I like the trailers

Because in Spanish, a real or likely future condition normally uses:

• si + present indicative, then
• a future tense or another suitable tense in the main clause

So this pattern is normal:

• Si tengo tiempo, iré. = If I have time, I’ll go.
• Si el tráiler no me gusta, elegiré... = If I don’t like the trailer, I’ll choose...

Spanish does not usually say si ... gustará here.

Because this sentence talks about a real, open possibility.

With si, Spanish normally uses the indicative for real conditions:

• Si viene, hablamos.
• Si no me gusta, elegiré otra.

You would use a different pattern for a hypothetical or unlikely situation:

• Si el tráiler no me gustara, elegiría otra película.

So here, since the speaker is talking about a normal possible future situation, gusta is correct.

Elegiré is the simple future of elegir.

The future tense is formed by adding endings to the infinitive:

• elegir
  • -é → elegiré

It is regular in the future tense, even though elegir has changes in some other forms, such as:

• elijo
• eliges

The accent mark in -é is part of the normal future ending:

• elegiré
• elegirá
• elegirán

Yes. Sentarse is a reflexive verb meaning to sit down.

So:

• me sentaré = I will sit down
• literally, I will seat myself

The pronoun changes with the subject:

• me sentaré
• te sentarás
• se sentará

This is different from sentar, which usually means something like to seat someone or to suit someone/something, depending on context.

Because in Spanish, otro/otra normally already carries the idea of another, so un/una is usually not added.

So:

• otra película = another film

This is the normal Spanish structure.

Compare:

• Quiero otra entrada. = I want another ticket.
• Necesito otra opción. = I need another option.

In this context, la cartelera means the cinema listings or the films currently showing.

So otra película de la cartelera means something like:

• another film from what’s on
• another film showing at the cinema
• another film in the listings

In Spain, cartelera is a very common word for what is currently being shown at the cinema, theatre, or on TV.

Más atrás means further back or more toward the back.

• atrás = back / behind
• más = more

So más atrás literally means more back, which English expresses as further back.

The order is normal in Spanish: más comes before the adverb it modifies.

Compare:

• más lejos = farther away
• más arriba = higher / further up
• más atrás = further back

Because Spanish has adapted the English word trailer to Spanish spelling and pronunciation as tráiler.

The accent mark shows the stress:

• trái-ler

This spelling is very common in Spanish, including in Spain, especially for a film preview.