Musimy zostać w domu, bo pada deszcz.

Musimy means we must or we have to.

It is the 1st person plural form of the verb musieć = to have to / must.

So:

• muszę = I must
• musisz = you must
• musi = he/she/it must
• musimy = we must

In this sentence, Musimy tells you that we are under some obligation.

Yes — zostać can mean different things depending on the context.

Here, zostać w domu means to stay at home / remain at home.

Compare:

• zostać w domu = stay at home
• zostać lekarzem = become a doctor

So the meaning changes depending on what comes after it:

• with a place: usually stay/remain
• with a profession/role/status: often become

In this sentence, because we have w domu (at home / in the house), the meaning is clearly stay.

This is about aspect, which is very important in Polish.

• zostać is perfective
• zostawać is imperfective

In a sentence like Musimy zostać w domu, Polish usually uses zostać to refer to a single concrete situation: we have to stay home now, today, in this situation.

By contrast, zostawać would suggest something more repeated, habitual, or ongoing in a broader sense. For example:

• Musimy zostać w domu dzisiaj. = We have to stay home today.
• Musimy zostawać w domu w weekendy. = We have to stay home on weekends.

So in your sentence, zostać is the natural choice.

Because w domu expresses location, not movement.

• w domu = in/at home
• do domu = to home / homeward

Compare:

• Musimy zostać w domu. = We have to stay at home.
• Musimy wrócić do domu. = We have to return home.

So:

• w
  • location = being somewhere
• do
  • destination = going somewhere

Since the sentence is about staying and not going, w domu is correct.

Domu is the locative singular form of dom.

After w when it means in or at a place, Polish normally uses the locative case.

So:

• dom = house / home
• w domu = in the house / at home

This is why you do not see the basic form dom after w in this sentence.

It can mean both, depending on context.

In this sentence, the most natural English translation is at home.

Literally, w domu is in the house/home, but Polish dom often covers both the physical idea of a house and the more personal idea of home.

Also, Polish has no articles like a or the, so w domu does not explicitly say in the house or in the home. You choose the best English version from context.

Bo means because.

It is very common in everyday Polish and sounds natural here.

You could also say:

• Musimy zostać w domu, ponieważ pada deszcz.

That also means We have to stay at home because it’s raining, but ponieważ sounds a bit more formal or written than bo.

So:

• bo = common, everyday because
• ponieważ = also because, but more formal

Because in standard Polish spelling, a clause introduced by bo is separated with a comma.

So:

• Musimy zostać w domu, bo pada deszcz.

That comma is the normal, correct punctuation.

Because Polish does not need a dummy subject like English it in weather expressions.

English says:

• It is raining

But Polish does not need that it. Instead, it says something more like:

• Rain is falling
• Deszcz pada
• or, in the common order here, Pada deszcz

So pada deszcz is the normal Polish way to express it’s raining.

Pada is the 3rd person singular present tense form of padać.

• padać = to fall; in weather contexts, to rain / to snow
• pada = falls / is falling / rains / is raining

In Polish, the present tense often covers both simple present and present continuous meanings that English separates:

• pada deszcz = it rains / it is raining

In this sentence, the natural translation is it is raining.

Yes, Polish word order is more flexible than English, but the version you have is the most neutral and natural.

• Musimy zostać w domu, bo pada deszcz.

You could also say:

• Bo pada deszcz, musimy zostać w domu.

This puts more emphasis on the reason first: Because it’s raining, we have to stay home.

So the meaning stays basically the same, but the emphasis changes.

Yes, often you can.

If the context is clear, Pada by itself can mean It’s raining.

But Pada deszcz is fully explicit and very clear for learners.

Compare:

• Pada. = It’s raining.
• Pada deszcz. = Rain is falling / It’s raining.

Both are correct; the second one simply states the weather more explicitly.