Wieczorem obejrzymy film w salonie.

Because wieczorem is a special time expression built from wieczór (evening).

Grammatically, it is the instrumental singular form of wieczór, but in this kind of sentence it functions adverbially, meaning during the evening / in the evening.

So:

• wieczór = evening as a noun
• wieczorem = in the evening

This is a very common Polish pattern with parts of the day and similar time words.

Usually, no.

For the general meaning in the evening, Polish normally says wieczorem, not w wieczór.

So:

• Wieczorem obejrzymy film. = correct
• W wieczór obejrzymy film. = unnatural in normal Polish

However, with an extra word, Polish can use other patterns, for example:

• w ten wieczór = on that evening
• w sobotni wieczór = on Saturday evening
• tego wieczoru = that evening

So the bare form wieczorem is the normal choice for a general statement.

Obejrzymy is the 1st person plural future form of the verb obejrzeć.

So:

• obejrzeć = the dictionary form / infinitive
• obejrzymy = we will watch / we will see

This verb is usually used for watching something as a complete event, especially something like a film, show, or video.

Because Polish often expresses future tense inside the verb form itself.

Here, obejrzymy already means we will watch. You do not need a separate word like English will.

That is especially true with perfective verbs such as obejrzeć. Their future tense is a simple one-word form:

• obejrzę = I will watch
• obejrzysz = you will watch
• obejrzymy = we will watch

So obejrzymy already contains both the future meaning and the subject information.

This is a classic Polish aspect difference.

• obejrzymy film uses a perfective verb
  → it presents the action as a whole, usually with the idea that the movie will be watched through to the end

• będziemy oglądać film uses an imperfective verb
  → it focuses more on the process of watching, without strongly emphasizing completion

So:

• Wieczorem obejrzymy film. = In the evening we’ll watch a film / we’ll watch the movie through
• Wieczorem będziemy oglądać film. = In the evening we’ll be watching a film

In many contexts both are possible, but they do not feel exactly the same.

It is built into the verb obejrzymy.

The ending -my shows 1st person plural, so Polish does not need a separate subject pronoun here.

That means:

• obejrzymy = we will watch

You could add my (we) if you want emphasis or contrast:

• My obejrzymy film, a oni pójdą spać.
  = We will watch a film, and they will go to sleep.

But in a neutral sentence, leaving my out is completely normal.

Film is the direct object of the verb, so it is in the accusative case.

But for many masculine inanimate nouns in singular, the accusative looks exactly the same as the nominative.

So:

• nominative: film
• accusative: film

That is why the form does not change.

Compare that with a masculine animate noun:

• kot = cat
• Widzę kota. = I see a cat

There, the accusative is different. But film is inanimate, so the form stays film.

Because w means in here, and it describes location, not motion.

After w meaning in, Polish normally uses the locative case. The locative singular of salon is salonie.

So:

• salon = nominative
• w salonie = in the living room

If you were talking about movement into the living room, Polish would more naturally say:

• do salonu = into/to the living room

So in this sentence:

• w salonie = location
• not w salon

Because Polish does not have articles like English a/an and the.

So film can mean:

• a film
• the film

The exact meaning depends on context.

In an isolated sentence like this, English often chooses a film because it sounds most neutral:

• Wieczorem obejrzymy film w salonie.
• In the evening we’ll watch a film in the living room.

But in the right context, it could also mean the film.

Yes. In home-related contexts, salon commonly means living room.

So w salonie here naturally means in the living room.

But salon can also mean other things in different contexts, for example:

• salon samochodowy = car showroom / car dealership
• salon fryzjerski = hair salon

So the word has a wider range than English living room, but in this sentence the meaning is clearly living room.

No. Polish word order is fairly flexible.

This sentence is natural and neutral, with Wieczorem at the front setting the time first. But other orders are also possible:

• Obejrzymy film wieczorem w salonie.
• Film obejrzymy wieczorem w salonie.
• W salonie obejrzymy film wieczorem.

These versions can sound slightly different in emphasis or focus, but they are still understandable.

The given order is a very normal way to present the sentence:

1. time: Wieczorem
2. action: obejrzymy
3. object: film
4. place: w salonie

Yes. Film is the standard Polish word for film/movie.

So in everyday Polish:

• obejrzeć film = to watch a film / movie
• dobry film = a good movie

Even though English sometimes distinguishes film and movie, Polish film is the normal everyday word for both ideas.