Placówka pocztowa jest zamknięta.

It means a postal branch, post office branch, or more literally a postal facility/outlet.

A few nuances:

• placówka = a branch, facility, establishment, outlet
• pocztowa = postal, related to the post

So this is a bit more formal and specific than just poczta. It sounds like the kind of wording you might see in official communication, notices, or customer information.

Because pocztowa is an adjective, and it describes placówka.

• poczta = post / mail / post office
• pocztowy / pocztowa / pocztowe = postal

In Polish, it is very common to use a noun plus an adjective like this:

• placówka pocztowa = postal branch
• sklep spożywczy = grocery store
• dworzec kolejowy = railway station

So pocztowa agrees with placówka, which is feminine singular.

Because it agrees with placówka, which is a feminine singular noun.

Polish adjectives change their endings to match the noun in:

• gender
• number
• sometimes case

Here:

• placówka = feminine singular
• so zamknięta must also be feminine singular

Compare:

• Sklep jest zamknięty. = The shop is closed.
  (sklep is masculine)
• Placówka jest zamknięta. = The branch/facility is closed.
  (placówka is feminine)
• Biuro jest zamknięte. = The office is closed.
  (biuro is neuter)

Jest means is.

So the structure is:

• Placówka pocztowa = the postal branch
• jest = is
• zamknięta = closed

Together: The postal branch is closed.

Unlike in some other Slavic languages, Polish normally does use the present-tense form of być (to be) in full standard sentences like this.

That said, in signs or notices, Polish often drops jest:

• Placówka pocztowa zamknięta.
• Urząd pocztowy zamknięty.

This sounds more like notice-board style. The full sentence with jest is completely normal and grammatical.

For a learner, the most useful way to think about it here is: it functions like an adjective meaning closed.

Historically/formally, forms like zamknięty / zamknięta / zamknięte come from a participial form of the verb zamknąć = to close. But in everyday use, in sentences like this, they behave very much like adjectives:

• Drzwi są zamknięte. = The door is closed.
• Sklep jest zamknięty. = The shop is closed.

So in practice, you can treat zamknięta here as the descriptive word closed, matched to a feminine noun.

Yes, often you can, but the meaning is a little less precise.

• Poczta jest zamknięta. = The post office is closed.
• Placówka pocztowa jest zamknięta. = The postal branch/facility is closed.

Poczta is more everyday and common in speech.
Placówka pocztowa sounds more official, administrative, or formal.

So if you are speaking casually, Poczta jest zamknięta is very natural. If you are reading a notice, website, or official message, Placówka pocztowa jest zamknięta fits well.

Yes, there is a small but useful difference.

• zamknięta = closed
• nieczynna = not open / out of service / not operating

For a post office, shop, or office, both can be used, but:

• zamknięta often suggests literally closed
• nieczynna is very common in public notices and business-hour contexts

Examples:

• Sklep jest zamknięty. = The store is closed.
• Apteka jest nieczynna. = The pharmacy is closed / not open.

On signs, nieczynne / nieczynna is extremely common in Polish.

Placówka pocztowa is in the nominative case, because it is the subject of the sentence.

Zamknięta also appears in nominative feminine singular because it agrees with the subject in this kind of sentence with być.

So:

• placówka — nominative singular feminine
• pocztowa — nominative singular feminine, agreeing with placówka
• zamknięta — nominative singular feminine, agreeing with placówka

This is one reason the endings match so neatly.

Yes. Polish word order is more flexible than English, although the basic order here is very natural.

Standard order:

• Placówka pocztowa jest zamknięta.

You may also see:

• Zamknięta jest placówka pocztowa.

This sounds more marked or emphatic, as if focusing on the fact that it is closed.

Because Polish uses endings to show grammatical relationships, word order often changes for emphasis, style, or information structure. But for a learner, the original order is the safest default.

A simple learner-friendly pronunciation guide:

• Placówka ≈ pla-TSOOF-ka
• pocztowa ≈ poch-TCHTO-va
• jest ≈ yest
• zamknięta ≈ zam-KNYEN-ta

A few pronunciation notes:

• Polish stress is usually on the second-to-last syllable
• ó sounds like u
• cz sounds like English ch, but a bit harder
• ę in zamknięta is often pronounced approximately like en before t

You do not need perfect phonetics at first; getting the stress and consonants roughly right is enough to be understood.

Overall, it sounds neutral to formal.

Why?

• placówka pocztowa is more official/formal than everyday poczta
• the full sentence with jest sounds complete and standard

You might hear more casual speech like:

• Poczta jest zamknięta.

You might see more notice-like wording such as:

• Placówka pocztowa nieczynna.
• Placówka zamknięta.

So the original sentence is perfectly natural, but it leans a bit toward formal or official usage.