Zostań u nas do niedzieli.

Yes, zostać can mean to become, but it also commonly means to stay / remain depending on context.

In Zostań u nas do niedzieli, it clearly means stay:

• zostań = stay / remain
• u nas = at our place / with us
• do niedzieli = until Sunday

So here it is not become anything; it is an invitation or request to remain somewhere.

Zostań is the imperative form of zostać for one person in the informal singular.

So it is what you would say to:

• one friend
• one family member
• one child
• one person you address with ty

Related forms:

• zostań = stay! (to one person, informal)
• zostańcie = stay! (to more than one person, or sometimes informal plural)
• a polite/formal version is usually expressed differently, for example: Niech pan/pani zostanie u nas do niedzieli.

This is about aspect, and it can feel unfamiliar to English speakers.

Zostać is perfective, and zostawać is imperfective.

In imperatives, Polish often uses the perfective form when asking someone to complete or carry out a whole action:

• Zostań u nas do niedzieli. = Stay with us until Sunday.

This sounds natural because the speaker is asking for a complete, bounded situation: remain here up to a certain point.

Zostawaj is much less natural here. It would sound unusual in this context.

So even though English just says stay, Polish naturally prefers zostań here.

U nas literally means something like at us, but in natural English it means:

• at our place
• with us
• in our home

It is a very common Polish expression when talking about being at someone’s home, workplace, or general environment.

Examples:

• Jestem u Marka. = I’m at Marek’s place.
• Będziesz u nas? = Will you be at our place?

So in this sentence, u nas means the person is being invited to stay with the speakers / at their home.

Because the preposition u takes the genitive case.

The pronoun my changes in the genitive:

• my = we
• nas = genitive form

So:

• u nas = at our place / with us

This is just the required case after u.

You can think of it as a fixed pattern:

• u mnie = at my place
• u ciebie = at your place
• u niego = at his place
• u nas = at our place

Because do takes the genitive case, and niedziela changes to niedzieli in the genitive singular.

So:

• nominative: niedziela = Sunday
• genitive: niedzieli

That is why we say:

• do niedzieli = until Sunday

This is a very common pattern:

• do poniedziałku = until Monday
• do wieczora = until evening
• do końca tygodnia = until the end of the week

This can be a little ambiguous, just like until Sunday can be in English.

Usually, do niedzieli means up to Sunday, and in real life it often suggests staying until Sunday arrives, or possibly through part of Sunday, depending on context.

If someone wants to be very clear, they may say:

• do niedzieli włącznie = through Sunday / including Sunday
• do końca niedzieli = until the end of Sunday

So do niedzieli is normal and natural, but context decides exactly how inclusive it is.

No, Polish word order is fairly flexible.

Zostań u nas do niedzieli is a very natural, neutral order.

But other orders are also possible, for example:

• Do niedzieli zostań u nas.
• U nas zostań do niedzieli.

These alternatives may shift the emphasis:

• Do niedzieli first emphasizes the time limit.
• U nas first emphasizes the place.

The original sentence is probably the most neutral and idiomatic.

Grammatically, it is an imperative, but in real use it can sound like:

• a warm invitation
• a suggestion
• a request
• encouragement

So Zostań u nas do niedzieli does not have to sound harsh like an order. Very often it means something like:

• Stay with us until Sunday.
• Why don’t you stay with us until Sunday?

The tone depends a lot on the situation and the speaker’s voice. In many contexts, it sounds friendly and hospitable.