W piątek jest wypłata, więc po pracy idę na zakupy.

Because w + the accusative case is a very common way to say on a day of the week in Polish.

• w piątek = on Friday
• w poniedziałek = on Monday
• w sobotę = on Saturday

So in this sentence, W piątek means On Friday.

Polish sometimes allows day names without w in certain contexts, but for learners, w + day of the week is the safest and most standard pattern to use.

Good question. Prepositions often do cause case changes, but sometimes the accusative form looks exactly the same as the nominative form.

Here:

• nominative: piątek
• accusative: piątek

So the form does change grammatically, but it just happens to look identical.

Compare that with:

• sobota → w sobotę
• niedziela → w niedzielę

There, you can clearly see the accusative ending change.

In this sentence, wypłata means payday, salary payment, or the day you get paid.

So:

• W piątek jest wypłata = Friday is payday / We get paid on Friday

The noun wypłata can also mean payment or payout in other contexts, but here it most naturally refers to getting your wages/salary.

Jest wypłata is a natural Polish way to say that payday happens or there is salary payment on that day.

It focuses on the event:

• W piątek jest wypłata = Friday is payday

If you said mam wypłatę, it could sound more like I have a payment / I receive a payout, which is possible in some contexts, but it is less neutral here.

So jest wypłata is a very idiomatic way to talk about payday.

Więc means so, therefore, or thus.

In this sentence:

• W piątek jest wypłata, więc po pracy idę na zakupy.
• It’s payday on Friday, so after work I’m going shopping.

It introduces the result or consequence of the first clause.

Polish word order is fairly flexible, but więc commonly appears near the beginning of the second clause, just like so in English.

After the preposition po in the sense of after, Polish uses the locative case.

So:

• praca = work
• po pracy = after work

This is a very useful phrase to remember as a chunk:

• po pracy = after work
• po obiedzie = after lunch
• po szkole = after school

So the ending -y in pracy comes from the locative singular.

Usually it just means after work, in the everyday sense of after the working day / after I finish work.

It does not usually emphasize the task being completed. It refers more naturally to the period after your workday ends.

So:

• po pracy idę na zakupy = after work, I’m going shopping

Because idę comes from iść, which describes going right now / on one specific occasion / in one direction.

Here the speaker means:

• After work, I’m going shopping
  as a specific planned action on that day.

Compare:

• idę na zakupy = I’m going shopping / I’m going to the shops (now or this time)
• chodzę na zakupy w soboty = I go shopping on Saturdays / I go shopping regularly

So idę fits a one-time or definite trip, while chodzę is more habitual or repeated.

Literally, it is something like for shopping or to shopping, but the natural English meaning is simply:

• go shopping

The expression iść na zakupy is a standard Polish phrase:

• iść na zakupy = to go shopping

Here:

• zakupy is a plural noun meaning shopping / purchases
• na is the preposition used in this set phrase

So it is best learned as a whole expression rather than translated word for word.

Here na takes the accusative case, because it expresses movement toward an activity or destination.

So:

• zakupy is the accusative plural form here

In practice, na zakupy is just the standard phrase used after verbs of motion:

• idę na zakupy = I’m going shopping
• jedziemy na zakupy = We’re going shopping

For learners, the most helpful thing is to memorize iść na zakupy as a fixed pattern.

In Polish, the idea of shopping is commonly expressed with the plural noun zakupy.

So even though English uses the uncountable noun shopping, Polish usually says:

• zakupy = shopping / purchases

Examples:

• robić zakupy = to do shopping
• iść na zakupy = to go shopping

This is just one of those vocabulary differences between the two languages.

Yes. That would also be grammatical, but the emphasis changes a little.

• W piątek jest wypłata, więc po pracy idę na zakupy.
  This first sets up Friday/payday, then gives the consequence.

• Po pracy w piątek idę na zakupy.
  This puts more focus on after work on Friday as the time of going shopping.

Polish word order is flexible, so different versions are possible depending on what you want to emphasize.

Yes — several very useful chunks:

• w piątek = on Friday
• jest wypłata = it’s payday
• więc = so / therefore
• po pracy = after work
• iść na zakupy = to go shopping

These are all common everyday expressions, so learning them as ready-made phrases will help a lot.