Posiadacz rachunku zastrzega kartę.

Because rachunku is in the genitive case.

The base form is rachunek = account / bill / invoice, but in the phrase posiadacz rachunku, Polish uses the genitive to mean holder of an account.

So:

• rachunek = account
• posiadacz rachunku = account holder

This is very common in Polish: one noun is followed by another noun in the genitive to show a relationship like of in English.

Examples:

• numer telefonu = phone number
• właściciel domu = owner of the house
• posiadacz rachunku = holder of the account

Posiadacz means holder or possessor.

It comes from the verb posiadać = to possess / to have / to hold.

In everyday speech, posiadacz can sound a bit formal or official. That fits this sentence well, because banking and legal language in Polish often uses more formal nouns like this.

So:

• posiadać = to possess, to hold
• posiadacz = holder, possessor

In a banking context, posiadacz rachunku is a standard formal phrase for account holder.

In everyday Polish outside banking, zastrzegać can mean to reserve, to stipulate, or to make a reservation/condition depending on context.

But in banking, zastrzec kartę / zastrzegać kartę has a specialized meaning: to block a card, usually because it was lost, stolen, or compromised.

So in this sentence, it does not mean that the account holder is reserving the card for later use. It means they are reporting it so it becomes blocked.

This is a good example of a verb whose meaning changes with context.

Because kartę is the accusative singular form of karta.

The verb zastrzega takes a direct object, and direct objects in Polish often appear in the accusative case.

So:

• karta = card (dictionary form / nominative)
• kartę = card (accusative)

Compare:

• Karta jest nowa. = The card is new.
  
  • karta is the subject, so nominative.
• Posiadacz zastrzega kartę. = The holder blocks the card.
  
  • kartę is the object, so accusative.

Zastrzega is present tense, third person singular.

It means:

• he reserves / blocks
• she reserves / blocks
• it reserves / blocks
• or, in this context, the account holder blocks the card

The infinitive is zastrzegać.

Conjugation:

• ja zastrzegam
• ty zastrzegasz
• on/ona/ono zastrzega
• my zastrzegamy
• wy zastrzegacie
• oni/one zastrzegają

In formal instructions or system messages, Polish often uses present tense where English might also use present tense or something more procedural.

Zastrzegać is imperfective.

Its usual perfective partner is zastrzec.

So:

• zastrzegać = to be blocking / to block in a general, repeated, or ongoing sense
• zastrzec = to block successfully as a completed action

Examples:

• Posiadacz zastrzega kartę. = The account holder is blocking the card / blocks the card.
• Posiadacz zastrzegł kartę. = The account holder blocked the card.

In a standalone sentence like this, the imperfective may be used in formal descriptions, procedures, or headings. In normal narration about a completed one-time event, Polish often prefers the perfective past:

• zastrzegł kartę

Yes, that is close, but account holder is the better translation.

Why?

• posiadacz = holder / possessor
• owner is usually właściciel

In banking, account holder is the standard English equivalent of posiadacz rachunku. Using owner is understandable, but slightly less precise.

So the best match is:

• posiadacz rachunku = account holder

Because Polish has no articles.

English distinguishes:

• a card
• the card

Polish does not have separate words for that. The meaning depends on context.

So:

• Posiadacz rachunku zastrzega kartę could mean The account holder blocks the card or, in a more general context, An account holder blocks a card.

In practice, context usually makes it clear.

Yes, you could, because Polish word order is more flexible than English.

The neutral order here is:

• Posiadacz rachunku zastrzega kartę.

That sounds like a plain, standard statement.

If you say:

• Kartę zastrzega posiadacz rachunku.

the meaning is still basically the same, but the emphasis changes. It puts more focus on the card or contrasts who is doing the blocking.

Because the cases show grammatical roles:

• posiadacz = subject
• kartę = object

Polish does not rely on word order as heavily as English does.

Yes, rachunek can mean several things depending on context:

• account
• bill
• check
• calculation

In this sentence, because of the banking phrase posiadacz rachunku and the mention of a card, it clearly means bank account.

Polish often uses context to narrow down meanings like this.

You may also see:

• konto = account

In everyday speech, many people would naturally say konto bankowe or just konto, but rachunek is also very common in official and banking language.

A rough English-friendly guide is:

po-SYA-dach ra-HOON-koo za-STSHEH-ga KAR-ten

A few useful notes:

• si before a sounds like a soft ś-like sound, so po-sia- is not a hard see-ah
• cz sounds like ch in chop
• rz usually sounds like zh as in measure
• szcz / strz / trz-type clusters can be hard at first, and zastrzega has that kind of consonant combination
• ę at the end of kartę is a nasal vowel, but in normal speech it is often heard approximately like en

Stress in Polish is usually on the second-to-last syllable:

• po-sia-DACZ
• ra-CHUN-ku
• za-STRZE-ga
• KAR-tę

It sounds formal and a bit institutional, which is normal for banking language.

A Polish speaker might say something simpler in everyday speech, for example:

• Zablokował kartę. = He/She blocked the card.
• Muszę zastrzec kartę. = I need to block the card.
• Posiadacz konta zastrzegł kartę. = The account holder blocked the card.

But in official documents, forms, procedures, or bank messages, Posiadacz rachunku zastrzega kartę sounds completely natural.