La cuenta atrás combina con la ambientación.

Yes. Cuenta atrás is a common fixed expression meaning countdown.

Literally:

• cuenta = count / counting
• atrás = back / backwards

So the original idea is counting backwards, which gives the meaning countdown.

In Spain, cuenta atrás is the normal everyday term. In many Latin American countries, you may also hear cuenta regresiva.

Example:

• Empieza la cuenta atrás. = The countdown begins.

Because the main noun is cuenta, and cuenta is feminine.

So:

• la cuenta
• therefore la cuenta atrás

Even though the whole expression means countdown, the gender still comes from cuenta.

Here, combina con means something like:

• goes well with
• matches
• fits with

So La cuenta atrás combina con la ambientación means the countdown fits the atmosphere/setting.

The verb combinar is often used for things that match visually, stylistically, or emotionally.

Examples:

• Ese color combina con la pared. = That color matches the wall.
• La música combina con la escena. = The music goes well with the scene.

Because combinar con is the usual pattern when you mean to match/go with something.

So:

• X combina con Y = X goes with Y

In your sentence:

• La cuenta atrás = the thing that matches
• con la ambientación = what it matches/goes with

If you leave out con, the meaning changes or the sentence becomes unnatural in this context.

Ambientación is a useful word that can mean:

• atmosphere
• setting
• ambience
• sometimes the way a scene/world is presented

The best translation depends on context.

For example, in a film, game, or story, ambientación can include:

• visuals
• sound
• mood
• setting
• overall style

So in this sentence, la ambientación is probably the overall mood or setting, not just the physical location.

Because Spanish often uses the present tense for a general description or opinion.

La cuenta atrás combina con la ambientación means:

• The countdown goes well with the atmosphere
• The countdown fits the setting

It sounds like a current judgment or general statement.

This is very normal in reviews, commentary, and descriptions.

Yes, often you can.

• combina con = a bit more neutral or descriptive
• pega con = more informal, very common in speech, especially in Spain

So:

• La cuenta atrás combina con la ambientación.
• La cuenta atrás pega con la ambientación.

Both are understandable, but combina con sounds a little more standard or polished.

The subject is La cuenta atrás.

Why? Because the verb is combina, and the thing doing the matching is la cuenta atrás.

Structure:

• La cuenta atrás = subject
• combina = verb
• con la ambientación = prepositional phrase

So literally:

• The countdown matches with the atmosphere

Yes. The plural is cuentas atrás.

Examples:

• Las cuentas atrás de las misiones son tensas.
• Hay varias cuentas atrás en la película.

Only cuenta changes to plural; atrás stays the same.

Yes, they are important.

• atrás
• ambientación

The written accents show the stress:

• a-TRÁS
• am-bien-ta-CIÓN

You should keep the accents in writing, because Spanish spelling treats them as part of the correct word.

Yes. It can refer to any countdown, including:

• a literal numerical countdown
• a timer
• a dramatic countdown in a film or game
• a metaphorical countdown to an event

So in this sentence, it could be a visual countdown, a narrative countdown, or even the general idea of time running out.