Из-за ливня мы остались дома и смотрели фильм.

Here из-за means because of or due to.

So Из-за ливня = because of the downpour.

It is a common Russian preposition used to show cause, often with something negative or inconvenient:

• из-за дождя — because of the rain
• из-за пробок — because of the traffic jams
• из-за ошибки — because of the mistake

It is written with a hyphen: из-за.

Because из-за requires the genitive case.

The dictionary form is ливень = downpour.
In the genitive singular, it becomes ливня.

So:

• ливень — nominative
• из-за ливня — genitive after из-за

This is something you will need to memorize with prepositions: many Russian prepositions always require a certain case.

Not exactly. Ливень means a heavy rain, downpour, or rainstorm.

Compare:

• дождь — rain
• ливень — a strong, pouring rain

So из-за ливня sounds stronger than из-за дождя. It suggests the weather was bad enough to affect plans.

Остались is the past tense plural of остаться.

In this sentence, остаться дома means to stay home or to remain at home.

So:

• мы остались дома = we stayed home

The verb остаться is perfective, so it presents the staying home as a result or decision:

• because of the downpour, we ended up staying home

Its imperfective partner is оставаться.

Смотрели is the imperfective past of смотреть.

In this sentence, imperfective fits well because it describes the activity as something they were doing while at home:

• we stayed home and watched a film

Russian often uses imperfective for background activities, processes, or simple narration of what someone did without emphasizing completion.

If you said посмотрели фильм, that would emphasize watching the film as a completed whole:

• Мы остались дома и посмотрели фильм = we stayed home and watched a film / finished watching a film

So the difference is roughly:

• смотрели фильм — were watching / watched
• посмотрели фильм — watched it to completion

This is a very natural Russian combination.

• остались (perfective) focuses on the result: they ended up staying home
• смотрели (imperfective) describes the activity that followed: they watched a film

So the sentence structure is:

1. a completed decision/result — we stayed home
2. an activity during that time — and watched a film

Russian often mixes aspects this way when one verb gives the outcome and the other gives the ongoing or descriptive action.

Дома means at home. It is the normal word Russians use for being at home.

So:

• мы остались дома = we stayed home

В доме literally means in the house/building. It usually refers more to physical location inside a house, not the general idea of being at home.

Compare:

• Я дома. — I’m at home.
• Я в доме. — I’m in the house.

In most everyday situations, if English says home, Russian uses дома.

Yes, Russian can often omit subject pronouns when the verb form already shows who the subject is.

So Остались дома и смотрели фильм could be understood from context as [we] stayed home and watched a film.

But мы is perfectly normal here. It can make the sentence clearer, more natural in isolation, or slightly emphasize we.

So:

• Мы остались дома и смотрели фильм. — very normal
• Остались дома и смотрели фильм. — also possible in the right context

Yes. In the past tense, Russian verbs agree in number, and in the singular they also agree in gender.

Since мы means we, the verb must be plural:

• мы остались
• мы смотрели

In the plural, there is no gender distinction:

• я остался — I stayed (male speaker)
• я осталась — I stayed (female speaker)
• мы остались — we stayed

The same applies to смотрели.

No, Russian word order is fairly flexible, although this version is the most neutral and natural:

Из-за ливня мы остались дома и смотрели фильм.

This order works well because:

• Из-за ливня sets up the reason first
• мы gives the subject
• остались дома и смотрели фильм gives the main actions

Other word orders are possible for emphasis, for example:

• Мы из-за ливня остались дома и смотрели фильм.
• Дома мы остались из-за ливня и смотрели фильм.

But the original sounds like a standard, neutral sentence.

No, not here.

The sentence has one subject, мы, and two coordinated past-tense verbs:

• остались
• смотрели

They are joined by и and share the same subject, so no comma is needed:

Из-за ливня мы остались дома и смотрели фильм.

A comma would only appear in different structures, for example if there were separate clauses or extra inserted information.

Grammatically, you can break it down that way, but that sounds too literal in natural English.

A better natural translation is:

• Because of the downpour, we stayed home and watched a film.

Russian and English do not always match word for word:

• остались дома is most naturally stayed home
• смотрели фильм is most naturally watched a film

So it is better to translate the whole sentence naturally rather than mechanically.