W niedzielę restauracja bywa zamknięta.

Because after w meaning on with a day or time expression, Polish often uses the accusative case.

So:

• niedziela = Sunday
• accusative singular: niedzielę
• w niedzielę = on Sunday

This is a very common pattern with days and parts of the week in time expressions.

It can often be understood as either, depending on context.

In this sentence, because of bywa, the sense is usually on Sundays / on a Sunday it tends to be. In other words, it suggests a repeated or typical situation, not just one specific Sunday.

So the whole sentence feels like:

• On Sundays, the restaurant is sometimes / tends to be closed.

Bywa comes from bywać, which means something like:

• to happen from time to time
• to be sometimes
• to tend to be
• to be on some occasions

So restauracja bywa zamknięta does not mean the restaurant is always closed. It suggests a habitual but not absolute situation.

A good English feeling for it is:

• The restaurant is sometimes closed
• The restaurant tends to be closed
• The restaurant can be closed

This is an important difference.

• jest zamknięta = is closed
  This sounds like a straightforward fact about its current state, or a general statement with no special idea of repetition or occasionality.

• bywa zamknięta = is sometimes closed / tends to be closed
  This adds the idea that it happens repeatedly, but not necessarily every time.

So:

• W niedzielę restauracja jest zamknięta.
  = On Sunday / On Sundays the restaurant is closed.

• W niedzielę restauracja bywa zamknięta.
  = On Sundays the restaurant is sometimes closed / tends to be closed.

Because restauracja is a feminine singular noun, and zamknięta agrees with it.

Polish adjectives and participle-like forms must match the noun in:

• gender
• number
• case

Here:

• restauracja = feminine singular
• so the form is zamknięta

Compare:

• sklep jest zamknięty = the shop is closed
  (sklep is masculine)

• restauracja jest zamknięta = the restaurant is closed
  (restauracja is feminine)

• okno jest zamknięte = the window is closed
  (okno is neuter)

It behaves like one here, yes.

Formally, zamknięta comes from the verb zamknąć and is a past passive participle, but in sentences like this it works very much like an adjective meaning closed.

So for a learner, it is practical to think of it as an adjective-like word that agrees with the noun:

• restauracja zamknięta
• drzwi zamknięte
• sklep zamknięty

There actually is a verb in the sentence: bywa.

Polish often uses a verb plus an adjective/participle to describe a state.

Here:

• restauracja = the restaurant
• bywa = is sometimes / tends to be
• zamknięta = closed

So bywa is doing the job that English often expresses with is plus an adverb like sometimes.

A very literal breakdown would be:

• The restaurant tends to-be closed on Sunday.

Yes, you may also hear w niedziele, and it usually means on Sundays more explicitly.

Very roughly:

• w niedzielę can mean on Sunday or, in the right context, on Sundays
• w niedziele clearly points to repeated Sundays: on Sundays

So:

• W niedzielę restauracja bywa zamknięta = natural, common
• W niedziele restauracja bywa zamknięta = also natural if you want to stress the repeated meaning

Both are possible, but the singular form is very common in Polish for general time expressions.

No, Polish word order is relatively flexible.

The original sentence is natural:

• W niedzielę restauracja bywa zamknięta.

But other orders are also possible, with slightly different emphasis:

• Restauracja bywa zamknięta w niedzielę.
  Neutral, also very natural.

• Restauracja w niedzielę bywa zamknięta.
  Slightly more focus on w niedzielę as part of the statement.

In many cases, Polish moves elements around for emphasis, rhythm, or style more than English does.

Yes, it is common and very useful.

Use bywać when you want to express:

• something that happens from time to time
• a repeated but not constant situation
• a habitual tendency

Examples:

• Bywam tam często. = I go there often / I am there often.
• On bywa zmęczony wieczorem. = He is sometimes tired in the evening.
• Restauracja bywa zamknięta w niedzielę. = The restaurant is sometimes closed on Sunday / on Sundays.

So bywać is helpful when plain być would sound too absolute.

Yes, a little.

Because of bywa, the sentence can suggest that this is not a fixed rule but rather something that happens often enough to mention.

It gives a softer, less absolute impression than a direct statement like:

• W niedzielę restauracja jest zamknięta.

So it can sound like:

• The restaurant is often closed on Sundays
• The restaurant may be closed on Sundays
• The restaurant tends to be closed on Sundays

That nuance of not always, but often enough is the key idea.