В субботу мы будем отдыхать дома и смотреть фильм.

Because Russian usually expresses on Saturday with в + accusative when talking about a day of the week.

• суббота = Saturday
• accusative singular = субботу
• so в субботу = on Saturday

This is a very common pattern:

• в понедельник = on Monday
• в среду = on Wednesday
• в пятницу = on Friday

So here в субботу is simply the normal way to say on Saturday.

Because отдыхать is an imperfective verb. In Russian, the future of an imperfective verb is formed with:

быть in the future + infinitive

So:

• будем = we will
• отдыхать = to rest / to relax

Together, будем отдыхать means we will be relaxing / we will relax.

This is called the compound future.

Compare:

• будем отдыхать = imperfective future, focusing on the process/activity
• отдохнём = perfective future, focusing more on the result or completion

In this sentence, будем отдыхать sounds natural because it describes planned activities for Saturday.

Because the same auxiliary verb будем applies to both infinitives:

• будем отдыхать
• (будем) смотреть

Russian often avoids repeating words when the meaning is already clear. So:

В субботу мы будем отдыхать дома и смотреть фильм.

means:

On Saturday we will relax at home and watch a film.

You could also say:

В субботу мы будем отдыхать дома и будем смотреть фильм.

But repeating будем is usually unnecessary here.

Because after будем, Russian uses the infinitive.

So:

• будем смотреть = we will watch

Not:

• будем смотрим ❌

As for посмотрим: that is a perfective future form meaning something like we will watch with a more completed/one-time sense.

Compare:

• будем смотреть фильм = we’ll be watching a film / we’ll watch a film
• посмотрим фильм = we’ll watch a film (as a completed event)

Both can be possible in different contexts, but this sentence uses the imperfective pattern.

Because дома is the usual word for at home.

• дома = at home
• в доме = in the house/building

So:

• мы будем отдыхать дома = we will relax at home

If you said в доме, it would sound more literal, as if you are emphasizing being inside a particular house rather than using the general idea of home.

Фильм is in the accusative case because it is the direct object of смотреть.

We watch what?
→ фильм

For many inanimate masculine singular nouns, the accusative looks exactly the same as the nominative.

So:

• nominative: фильм
• accusative: фильм

That is why the form does not change.

Compare with another inanimate masculine noun:

• Я читаю журнал. = I am reading a magazine.

But with animate masculine nouns, accusative usually changes differently.

Yes, often it could.

Russian verb forms usually show the person and number, so будем already tells you we will.

So both are possible:

• В субботу мы будем отдыхать дома и смотреть фильм.
• В субботу будем отдыхать дома и смотреть фильм.

Including мы can make the sentence clearer, more natural in some contexts, or slightly more emphatic.

It depends on context.

В субботу can mean:

• this Saturday / on Saturday in a specific situation
• on Saturdays only in some broader contexts, though for repeated habits Russian often uses other patterns more clearly

In your sentence, it most naturally means this coming Saturday or on Saturday as a specific plan.

If you wanted to make repeated meaning clearer, you might say something like:

• По субботам мы отдыхаем дома и смотрим фильмы. = On Saturdays we relax at home and watch films.

Yes, Russian word order is flexible.

The sentence:

В субботу мы будем отдыхать дома и смотреть фильм.

is a very neutral, natural order:

1. time: В субботу
2. subject: мы
3. verb phrase: будем отдыхать
4. place: дома
5. second action: и смотреть фильм

You could move parts around for emphasis, for example:

• Мы будем в субботу отдыхать дома и смотреть фильм.
• Дома мы будем отдыхать в субботу и смотреть фильм.

These are grammatically possible, but the original version sounds the most straightforward.

Because Russian has no articles.

So фильм can mean:

• a film
• the film

The exact meaning depends on context.

In this sentence, English might translate it as either:

• watch a film
• watch the film

If the meaning is already known from context, Russian usually does not need any extra word.

They are different aspects:

• отдыхать = imperfective
• отдохнуть = perfective

In the future:

• будем отдыхать = we will be relaxing / we will relax
• отдохнём = we will have a rest / we will relax fully

The sentence uses будем отдыхать because it presents relaxing as an activity taking place on Saturday, alongside смотреть фильм.

If you said:

• В субботу мы отдохнём дома и посмотрим фильм.

that would sound a bit more like a set of completed actions.

Not necessarily.

будем отдыхать дома и смотреть фильм means both activities are part of the plan for Saturday. In real life, watching a film can also be part of relaxing, so the actions fit together naturally.

Russian и simply means and here. It does not force a strict meaning like:

• exactly simultaneous
• one after the other

It just tells you both actions are included. Context decides the rest.