Welchen Film schauen wir heute Abend?

Because Film is the direct object of the sentence, so it has to be in the accusative case.

• der Film = nominative
• den Film = accusative

The question word welch- changes its ending to match the noun’s gender, number, and case. Since Film is masculine singular accusative, the correct form is welchen.

Compare:

• Welcher Film ist neu? = Which film is new?
  
  • Film is the subject, so nominative.
• Welchen Film schauen wir? = Which film are we watching?
  
  • Film is the object, so accusative.

Welchen is the accusative masculine form of welcher, which means which.

It works like a question word that also behaves a bit like an article. Its form changes depending on the noun that follows.

For example:

• welcher Film = which film
• welche Serie = which series
• welches Buch = which book
• welche Filme = which films

In your sentence, welchen matches Film because Film is masculine singular accusative.

In a German wh-question, the question word or question phrase comes first, and the finite verb usually comes second.

So the structure is:

• Welchen Film = question phrase
• schauen = verb
• wir = subject
• heute Abend = time expression

That gives:

Welchen Film schauen wir heute Abend?

This is different from normal statement word order:

• Wir schauen heute Abend einen Film.

In German questions with who/what/which/when, the verb still takes the second position after the opening question element.

Because this is a question, and in German the conjugated verb comes very early—typically in second position in wh-questions.

So even though wir is the subject, it comes after the verb here:

• Welchen Film schauen wir heute Abend?

This is normal German word order.

Compare:

• Statement: Wir schauen heute Abend einen Film.
• Question: Welchen Film schauen wir heute Abend?

Yes, you can often say sehen too.

• einen Film schauen = to watch a film
• einen Film sehen = to see/watch a film

In everyday German, schauen is very common, especially in spoken language. sehen is also correct, but schauen often feels a bit more natural for actively watching TV or a movie.

You may also hear:

• Welchen Film gucken wir heute Abend?

gucken is even more colloquial than schauen.

Because Abend is a noun, and all nouns are capitalized in German.

So:

• heute Abend = this evening / tonight

Even though English would normally write evening in lowercase, German keeps the noun capitalized.

It can mean either, depending on context.

• heute Abend literally means this evening
• In many contexts, natural English would translate it as tonight

So this sentence could naturally be understood as:

• Which film are we watching this evening?
• Which film are we watching tonight?

Both are fine.

German often uses a simple time expression without a preposition where English uses a special word like tonight.

So:

• heute Abend = tonight / this evening
• heute Morgen = this morning
• heute Nacht = tonight / during the night

You do not need a preposition like on or in here.

Yes.

• heute Abend = this evening / tonight, referring to today
• am Abend = in the evening, more general

Examples:

• Welchen Film schauen wir heute Abend?
  = Which film are we watching tonight?
• Am Abend schauen wir oft Filme.
  = In the evening we often watch films.

So heute Abend is specific, while am Abend is more general.

Yes. That would also be correct, but it means something slightly different in feel.

• Welchen Film ... ? = Which film ... ?
  This usually suggests a choice from known options.
• Was für einen Film ... ? = What kind of film ... ? / What film ... ?
  This can ask more about the type or nature of the film.

So if you are choosing between specific movies, Welchen Film is the more direct and natural choice.

The form of welch- changes to match the noun.

For example:

• Masculine: Welchen Film schauen wir?
• Feminine: Welche Serie schauen wir?
• Neuter: Welches Programm schauen wir?
• Plural: Welche Filme schauen wir?

This is a very useful pattern to learn, because it shows how German question words can reflect case and gender.

Yes. The standard grammatical gender is masculine:

• der Film

So in the accusative it becomes:

• den Film

And that is why the question phrase is:

• welchen Film

If you know the noun’s gender, it becomes much easier to choose the correct form of welcher.

A yes/no question in German starts with the verb.

So instead of:

• Welchen Film schauen wir heute Abend?
  = Which film are we watching tonight?

you could have:

• Schauen wir heute Abend einen Film?
  = Are we watching a film tonight?

That shows the difference between:

• wh-question: question word first
• yes/no question: verb first