Dla psa kupuję nową karmę, a dla kota nowy żwirek.

Dla means for. It always requires the genitive case.

So:

• pies → psa
• kot → kota

That is why the sentence has:

• dla psa = for the dog / for a dog
• dla kota = for the cat / for a cat

The basic dictionary forms are pies and kot, but after dla they must change.

Both of these are the things being bought, so they are in the accusative case after kupuję.

But Polish accusative looks different depending on gender:

• nowa karma → nową karmę
  feminine singular changes in the accusative

• nowy żwirek → nowy żwirek
  masculine inanimate singular usually stays the same in the accusative as in the nominative

So the difference comes from the gender and declension pattern of the noun:

• karma is feminine
• żwirek is masculine inanimate

Because the verb is understood in the second part. This is very normal in Polish.

The full version would be:

Dla psa kupuję nową karmę, a dla kota kupuję nowy żwirek.

But repeating kupuję is unnecessary, so it is often omitted. English does the same kind of thing sometimes:

• For the dog I’m buying new food, and for the cat new litter.

Not exactly.

• i usually means simple and
• a often connects two parallel or contrasting pieces of information

In this sentence, a sounds like:

• and as for
• while for
• and for

So the sentence has a nice comparison:

• for the dog, one thing
• for the cat, another thing

Using i would not be impossible in every context, but a is more natural here because the sentence sets the two animals side by side.

No. In this sentence karma means pet food or animal feed.

So:

• karma dla psa = dog food
• karma dla kota = cat food

This is a very common word in Polish, and it is unrelated in meaning to the English idea of spiritual karma.

Here żwirek means cat litter.

The word is based on żwir, which means gravel, so literally it has that kind of sandy/gravelly idea. But in everyday Polish, żwirek is a normal word for the litter you put in a cat’s litter tray.

So in this sentence:

• nowy żwirek = new cat litter

Because Polish does not have articles.

So dla psa can mean:

• for the dog
• for a dog

And dla kota can mean:

• for the cat
• for a cat

Context tells you which meaning is intended. This is normal in Polish.

No. Polish word order is fairly flexible.

The version you have:

Dla psa kupuję nową karmę, a dla kota nowy żwirek.

puts emphasis on who each thing is for.

Other possible orders include:

• Kupuję dla psa nową karmę, a dla kota nowy żwirek.
• Nową karmę kupuję dla psa, a nowy żwirek dla kota.

These all mean roughly the same thing, but the focus changes slightly. Starting with dla psa makes the recipient especially prominent.

Yes, you often can.

For example:

• Kupuję psu nową karmę, a kotu nowy żwirek.

This uses the dative case:

• psu
• kotu

That version is also natural and means something like I’m buying the dog new food and the cat new litter.

Very roughly:

• dla + genitive highlights for
• dative treats the animal more like an indirect object

Both patterns are common, and both are useful to know.

Because in Polish, a comma is normally used before conjunctions like a, ale, and several others.

So:

• Dla psa kupuję nową karmę, a dla kota nowy żwirek.

The comma is standard punctuation here. This is different from English, where comma use before and is more flexible. In Polish, before a in a sentence like this, the comma is expected.