Zupa gotuje się na kuchence, a ja kroję cebulę.

Because gotować się is the usual Polish verb for something cooking / boiling on its own.

• gotować = to cook something
• gotować się = to be cooking / to boil

So:

• Gotuję zupę = I am cooking soup
• Zupa gotuje się = The soup is cooking / boiling

The się here is very common in Polish and often makes the verb more like an intransitive or self-contained action. In English, we usually do not translate it word for word.

In this sentence, się does not literally mean itself in the natural English sense. It is part of the verb gotować się.

A useful way to think about it is:

• gotować = to cook something
• gotować się = to cook / boil

So się helps shift the meaning from someone cooks something to something is cooking.

This is very common in Polish:

• Drzwi otwierają się = The door opens
• Woda gotuje się = The water is boiling

Because Polish normally says something is cooking on a stove, not in a stove.

• na kuchence = on the stove / on the cooker

The preposition na often corresponds to English on.

Also, kuchence is the locative form of kuchenka, used after na when it means location.

Compare:

• na stole = on the table
• na półce = on the shelf
• na kuchence = on the stove

Because after na in a location meaning, Polish uses the locative case.

The dictionary form is:

• kuchenka = stove, cooker, hotplate

After na, it becomes:

• na kuchence

This is a normal pattern for many feminine nouns ending in -a:

• książka → w książce
• szafka → w szafce
• kuchenka → na kuchence

Because cebulę is in the accusative case, which is used for the direct object.

The basic form is:

• cebula = onion

But when it is the thing being cut, it becomes:

• kroję cebulę = I am cutting an onion / the onion

This is a very common pattern for feminine nouns ending in -a:

• mam książkę
• widzę kobietę
• kroję cebulę

So -a often changes to -ę in the accusative singular.

Because kroję is the correct 1st person singular present tense form of kroić (to cut).

Conjugation:

• ja kroję = I cut / I am cutting
• ty kroisz
• on/ona kroi
• my kroimy
• wy kroicie
• oni/one kroją

This verb has a stem change in the present tense, so the forms are not built as simply as kroi- + ending.

Yes, it could be omitted.

Polish often leaves out subject pronouns because the verb ending already shows who the subject is.

So both are possible:

• a ja kroję cebulę
• a kroję cebulę

Including ja gives extra emphasis or contrast. In this sentence, it sounds natural because there is a contrast between two simultaneous things:

• The soup is cooking, and I am cutting the onion

So ja helps highlight what I am doing.

Because a often links two ideas with a mild contrast or a side-by-side comparison, while i is a more basic and.

Here, a works well because the sentence sets two actions next to each other:

• Zupa gotuje się na kuchence, a ja kroję cebulę.

It is like:

• The soup is cooking on the stove, while I’m cutting the onion.

If you used i, it would still be understandable, but a sounds more natural for this kind of contrastive meanwhile / while I... connection.

Yes. They are both present tense imperfective forms, and here they describe actions happening right now.

• gotować się is imperfective
• kroić is imperfective

In Polish, the present tense of imperfective verbs can mean:

• a general/habitual action, or
• an action happening now

In this sentence, the context clearly gives the right now meaning.

So Polish does not need a special form exactly like English is cooking or am cutting. The ordinary present tense does that job.

It can be either, depending on context.

With zupa, woda, and similar things, gotować się often feels close to to boil or to be simmering.

Examples:

• Woda gotuje się. = The water is boiling.
• Zupa gotuje się. = The soup is cooking / boiling / simmering.

So the exact English translation depends on what sounds most natural in context.

się is pronounced roughly like shyeng or sheng, though neither is exact.

A few useful points:

• si before a vowel often sounds like a soft ś
• ę is a nasal vowel, though in normal speech its pronunciation can vary
• in fast everyday speech, się is often pronounced a bit more simply than the spelling suggests

You do not need to pronounce it in an exaggerated way. A natural, soft pronunciation is best.

No. They are different words.

• kuchnia = kitchen
• kuchenka = stove, cooker, hotplate

So:

• w kuchni = in the kitchen
• na kuchence = on the stove

This distinction is important because English learners sometimes confuse them due to the similar spelling.

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

Original:

• Zupa gotuje się na kuchence, a ja kroję cebulę.

Possible variations:

• Na kuchence zupa gotuje się, a ja kroję cebulę.
• Zupa na kuchence gotuje się, a ja kroję cebulę.

These alternatives are possible, but they shift emphasis. The original version is best for a normal, unmarked statement.

So for a learner, the safest choice is the original order:

• subject + verb + other information