Погода испортилась, и мы остались дома.

Because the subject is погода, which is:

• feminine
• singular
• in the past tense

In Russian past tense, verbs agree with the subject in gender and number.

So:

• испортился = masculine
• испортилась = feminine
• испортилось = neuter
• испортились = plural

Since погода is feminine singular, the correct form is испортилась.

It comes from испортиться, which literally means something like:

• to get spoiled
• to go bad
• to become ruined / worse

With погода, it means the weather got worse or the weather turned bad.

So Russian is not expressing this as the weather became bad with стала плохой, but with a very natural verb meaning it deteriorated.

The -ся ending often makes a verb reflexive, but in many cases it does not mean a literal English reflexive like herself or itself.

Here, испортить means to spoil something:

• Он испортил праздник = He spoiled the celebration.

But испортиться means to become spoiled / go bad:

• Молоко испортилось = The milk went bad.
• Погода испортилась = The weather got worse.

So -ся here turns the verb into an intransitive one: the weather is not spoiling something else; the weather itself changes state.

This is an aspect question.

• портиться = imperfective
• испортиться = perfective

In this sentence, the speaker is talking about a completed change: the weather changed and became bad. That is why испортиться is used.

Compare:

• Погода портилась весь день = The weather was getting worse all day.
  This focuses on the process.
• Погода испортилась = The weather got worse.
  This focuses on the result.

So the perfective verb fits the idea of a single completed event.

Because the subject is мы, which is plural.

Russian past tense verbs agree with the subject in number, and in the plural there is just one form:

• я остался / осталась
• ты остался / осталась
• он остался
• она осталась
• оно осталось
• мы остались
• вы остались
• они остались

So мы остались means we stayed / remained.

Again, this is aspect.

• оставаться = imperfective
• остаться = perfective

Остались presents the action as a single whole result: we ended up staying home.

Compare:

• Мы оставались дома весь день = We were staying home all day / We stayed home all day.
  This emphasizes duration or ongoing situation.
• Мы остались дома = We stayed home / We remained at home.
  This emphasizes the outcome or decision/result.

In this sentence, the meaning is that the weather worsened, and as a result, we stayed home. So the perfective works naturally.

Because дома is a very common word meaning at home.

So:

• Мы остались дома = We stayed home / We stayed at home.

By contrast:

• в доме literally means in the house/building

That usually refers more to the physical structure, not the idea of home in the everyday sense.

So:

• дома = at home
• в доме = inside the house

In this sentence, дома is the natural choice.

For practical learning purposes, it is best to treat дома here as a fixed adverb meaning at home.

Historically, it is related to a case form of дом, but in modern Russian learners usually just memorize:

• дома = at home
• домой = homeward, to home
• дом = house/home

So in this sentence, you do not need to analyze дома the same way you would analyze a normal noun in a regular case pattern. Just learn it as the standard word for at home.

This is a very common question.

• дома = at home → location
• домой = home / to home → direction

Compare:

• Мы остались дома = We stayed at home.
• Мы пошли домой = We went home.

A good shortcut:

• if there is movement toward home, use домой
• if you are already there / staying there, use дома

Because this sentence has two separate clauses:

1. Погода испортилась
2. мы остались дома

Each clause has its own subject and verb, so Russian normally puts a comma before и when it joins two independent clauses.

So the comma is standard and correct here.

Yes. The sentence literally says:

• The weather got worse, and we stayed home.

But in context, native speakers naturally understand a cause-and-result connection: the weather worsened, so we stayed home.

Russian often leaves that relationship implicit when it is obvious.

If you wanted to make the causal link more explicit, you could say:

• Погода испортилась, поэтому мы остались дома = The weather got worse, so we stayed home.
• Из-за плохой погоды мы остались дома = We stayed home because of the bad weather.

But the original sentence sounds completely natural.

Yes, Russian word order is flexible, though not random.

The original:

• Погода испортилась, и мы остались дома.

is a very neutral, natural order.

You could change it for emphasis, for example:

• Мы остались дома, потому что погода испортилась.
• Из-за того, что погода испортилась, мы остались дома.

But in the original sentence, the order first presents the situation, then the result. That is very natural storytelling order in Russian.

Normally, no. In standard Russian, you would usually keep мы here.

Russian can sometimes omit subjects, but much less freely than languages like Spanish or Italian. In this sentence, after и, saying мы остались дома is the normal and clear version.

So:

• Погода испортилась, и мы остались дома. = natural
• Погода испортилась, и остались дома. = incomplete or unnatural in standard usage unless the subject is very strongly understood from context and the style is conversational or elliptical