Jeśli sprzedawca ma tylko jedną porcję sernika, biorę ją dla babci, bo ona bardzo lubi słodkie rzeczy.

Some Polish masculine nouns, especially nouns for people and professions, end in -a. Sprzedawca is one of them. Even though the ending looks feminine, the word is grammatically masculine.

You can see that in agreement:

• ten sprzedawca = this seller
• dobry sprzedawca = a good seller

If you want to refer specifically to a female seller, Polish often uses sprzedawczyni.

Jeśli means if. It introduces a condition.

Jeśli and jeżeli usually mean the same thing. Jeżeli can sound a little more formal or careful, while jeśli is very common in everyday speech.

So:

• Jeśli sprzedawca ma... = If the seller has...
• Jeżeli sprzedawca ma... = same basic meaning

Polish uses commas to separate subordinate clauses.

In this sentence:

• Jeśli sprzedawca ma tylko jedną porcję sernika, ...
  The comma marks the end of the if clause.
• ..., bo ona bardzo lubi słodkie rzeczy.
  The comma before bo is standard, because bo introduces a reason clause.

So the commas are there because the sentence contains two dependent parts:

1. a condition
2. a reason

Because this phrase is the direct object of ma.

The verb mieć usually takes the accusative case:

• jedna porcja = nominative, the dictionary-style subject form
• jedną porcję = accusative, used after ma

Both words change because they agree with each other:

• jedna → jedną
• porcja → porcję

This is feminine singular accusative.

Because porcja is followed by the genitive: porcja czego? = a portion of what?

So:

• porcja sernika = a portion of cheesecake
• kawałek chleba = a piece of bread
• szklanka wody = a glass of water

This is very common in Polish. After words like porcja, kawałek, szklanka, butelka, the next noun is often in the genitive.

Here tylko means only, and it modifies jedną porcję sernika.

So:

• ma tylko jedną porcję sernika = has only one portion of cheesecake

Its position matters. If you move tylko, the emphasis can change:

• Tylko sprzedawca ma... = Only the seller has...
• Sprzedawca ma tylko jedną... = The seller has only one...

In the original sentence, the natural meaning is that the quantity is limited.

Biorę is the present tense of the imperfective verb brać.
Wezmę is the future of the perfective verb wziąć.

In this sentence, biorę can sound like:

• a habitual reaction
• a general rule
• a vivid, conversational present

So it is a bit like saying: if that situation happens, I take it for Grandma.

If you want to talk about one specific future situation, Polish would very often use:

• Jeśli sprzedawca będzie miał tylko jedną porcję sernika, wezmę ją dla babci.

That version sounds more clearly like a one-time future event.

Ją means her/it in the accusative, and here it means it. It refers back to porcję.

That is important: it refers to porcję because porcja is feminine:

• porcja → ją

It does not refer directly to sernik, because the thing being taken is the portion, not just the cheesecake as an abstract noun.

So:

• biorę ją = I take it

Dla means for, in the sense of doing something for someone’s benefit. After dla, Polish uses the genitive case.

So:

• babcia = grandmother / grandma
• dla babci = for Grandma

This construction is very common:

• dla mamy = for Mom
• dla kolegi = for a friend
• dla dzieci = for the children

Here it shows who the cheesecake portion is intended for.

Polish often omits subject pronouns, because the verb ending already shows the person. But here ona is useful for clarity.

Without ona, the sentence could feel less clear, because there are several people already mentioned:

• the seller
• the speaker
• Grandma

Bo ona bardzo lubi słodkie rzeczy clearly means because she really likes sweet things, and she is Grandma.

So ona is not absolutely required in every context, but here it helps avoid ambiguity and adds a bit of emphasis.

Słodkie rzeczy literally means sweet things. It is a broad, natural expression.

Słodycze means sweets, candy, or confectionery, which is more specific.

So:

• lubi słodkie rzeczy = she likes sweet things
• lubi słodycze = she likes sweets/candy

Both can work, but słodkie rzeczy is a bit wider in meaning. It can include cake, dessert, pastries, and other sweet foods, not just candy.

Here rzeczy is accusative plural, because lubić takes a direct object in the accusative:

• lubić coś = to like something

The noun is:

• rzecz = thing
• rzeczy = things

In the plural, rzeczy is the same form for nominative and accusative, so the form itself does not change here, but the function is accusative.

Also, słodkie matches rzeczy in number, gender, and case:

• słodkie rzeczy = sweet things

Yes, Polish word order is flexible, but the original order is neutral and natural.

The sentence:

• Jeśli sprzedawca ma tylko jedną porcję sernika, biorę ją dla babci, bo ona bardzo lubi słodkie rzeczy.

sounds like a normal, unmarked way to say it.

You can move things for emphasis, for example:

• Jeśli sprzedawca ma tylko jedną porcję sernika, dla babci biorę ją...
  This puts extra emphasis on for Grandma.
• Bo ona bardzo lubi słodkie rzeczy, biorę ją dla babci.
  This foregrounds the reason.

But not every rearrangement sounds equally natural. Polish word order is flexible, not random. The original version is a very good model to learn from.