Po blokadzie karty muszę zastrzec kartę.

Because po here means after, and in that meaning it requires the locative case.

• blokada = nominative singular
• blokadzie = locative singular

So:

• po blokadzie = after the block / after blocking

This is a very common pattern in Polish:

• po pracy = after work
• po obiedzie = after lunch
• po spotkaniu = after the meeting

Here karty is in the genitive singular.

The phrase blokada karty literally means blocking of the card or card block. In Polish, nouns often take another noun in the genitive to show what they relate to.

So:

• blokada = block / blocking
• karty = of the card

Compare:

• numer konta = account number
• właściciel domu = owner of the house
• koniec filmu = end of the film

Because kartę is the accusative singular, and it is the direct object of zastrzec.

• karta = nominative singular
• kartę = accusative singular

Since the sentence says what I must do to the card, Polish uses the accusative:

• muszę zastrzec kartę = I must block/cancel/report the card

This is very normal with transitive verbs:

• mam książkę = I have a book
• widzę kobietę = I see a woman
• kupuję kawę = I’m buying coffee

Muszę means I must or I have to.

The subject I is not written because Polish usually does not need a separate pronoun when the verb ending already shows the person.

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

So ja is optional here. You could say:

• Ja muszę zastrzec kartę.

But usually ja is omitted unless you want emphasis or contrast.

Because zastrzec is perfective. It refers to a complete, one-time action: finishing the act of blocking/canceling/reporting the card.

With muszę, Polish often uses a perfective infinitive when the meaning is I need to get this done.

• muszę zastrzec kartę = I must block/cancel the card completely

By contrast, zastrzegać is imperfective, which would sound more like an ongoing or repeated action. In this context, the completed action is what matters.

This perfective/imperfective contrast is very important in Polish.

They are very close, but they are not always identical.

• blokada karty often refers to a block placed on the card, sometimes temporary or technical
• zastrzec kartę usually means to report the card as lost/stolen and have it permanently stopped/cancelled

In banking language, zastrzec kartę is often stronger and more final than just zablokować kartę.

So the sentence may suggest a sequence like:

1. first there is a block
2. then I still need to officially report/cancel the card

That is why the sentence can sound a bit more specific than a simple repetition.

No, Polish word order is fairly flexible, although some versions sound more neutral than others.

The given sentence:

• Po blokadzie karty muszę zastrzec kartę.

is natural and neutral.

You could also hear:

• Muszę zastrzec kartę po blokadzie karty.
• Kartę muszę zastrzec po blokadzie karty.
  This puts extra focus on kartę.

Because Polish uses case endings, word order often changes emphasis rather than basic meaning.

Yes: it is an irregular-looking verb and can be hard to recognize in different forms.

The infinitive is:

• zastrzec

But related forms look quite different, for example:

• zastrzegam = I reserve / I stipulate
• zastrzeżony = reserved / blocked
• zastrzeżenie = reservation / objection / stipulation

In banking, you may also see:

• karta została zastrzeżona = the card has been blocked/cancelled

So it is a useful verb to learn as a whole expression:

• zastrzec kartę

rather than trying to build it from parts at first.

A rough English-friendly pronunciation is:

• ZA-stshets

A few tricky points:

• rz is usually pronounced like ż
• strz is a difficult consonant cluster
• the final -ec here sounds roughly like -ets

If you slow it down, it helps:

• za-strzec

It is one of those words that may be easier to recognize in context before it becomes easy to say.

Yes, but the structure would change.

• Po blokadzie karty... = after the card block / after blocking the card
  This is more compact and noun-based.
• Po tym jak zablokuję kartę... = after I block the card
  This is clause-based and more explicit.

So both can work, but they are built differently:

• po + noun in locative
• po tym jak + verb clause

The original sentence uses the shorter, more formal-sounding noun phrase.

It sounds fairly neutral, possibly a little official because of the banking vocabulary.

Expressions like:

• blokada karty
• zastrzec kartę

are very common in customer service, banking, and formal instructions.

In casual speech, a person might still say this, but they might also choose a simpler wording depending on context, especially if the listener already understands what kind of card action they mean.