Kiełbasę i chleb kroję na desce, a potem robię szybkie kanapki.

Because it is the direct object of kroję (I cut / I am cutting).

Kiełbasa is a feminine singular noun, and feminine singular nouns usually change in the accusative case:

• kiełbasa = nominative
• kiełbasę = accusative

So Kiełbasę i chleb kroję... means that sausage and bread are the things being cut.

Because chleb is a masculine inanimate noun. In Polish, the accusative singular of masculine inanimate nouns is usually the same as the nominative singular.

So:

• chleb = nominative
• chleb = accusative

That is why you get:

• kiełbasę changes
• chleb does not

This is very common in Polish and can feel strange to English speakers at first, because English does not mark these object forms on nouns.

Kroję is the 1st person singular present tense form of kroić (to cut / to be cutting).

It means:

• I cut
• I am cutting

Polish present-tense verb endings usually show the subject clearly, so you do not need to say ja.

Examples:

• kroję = I cut
• kroisz = you cut
• kroi = he/she/it cuts

So kroję by itself already tells you the subject is I.

Because Polish often omits subject pronouns when they are clear from the verb ending.

Here:

• kroję = I cut
• robię = I make

So adding ja would usually be unnecessary unless you want emphasis, contrast, or extra clarity.

For example:

• Kroję chleb. = I am cutting bread.
• Ja kroję chleb. = I am the one cutting bread / As for me, I’m cutting bread

Because na can require different cases depending on meaning.

Here na desce means on the board, so it expresses location. With na meaning location, Polish uses the locative case.

So:

• deska = nominative
• na desce = locative

This is the same pattern as:

• na stole = on the table
• na podłodze = on the floor

If na expressed movement onto something, the case would usually be different.

A potem means something like:

• and then
• then
• and after that

The conjunction a often links two clauses in a natural, conversational way. It is not exactly the same as i, even though both can often be translated as and.

In this sentence:

• ..., a potem robię szybkie kanapki.

it sounds very natural: first one action happens, then another.

Using i potem is possible in some contexts, but a potem is very common when moving the story forward from one action to the next.

Szybkie kanapki is the direct object of robię (I make), so it is in the accusative.

However, for non-masculine-personal plural nouns like kanapki, the accusative plural looks the same as the nominative plural.

So:

• kanapki = nominative plural
• kanapki = accusative plural

And the adjective agrees with the noun:

• szybkie kanapki

That is why nothing visibly changes here, even though the phrase is functioning as the object.

Because adjectives in Polish must agree with the noun in gender, number, and case.

Here the noun is:

• kanapki = plural, non-masculine-personal, accusative

So the adjective must match:

• szybkie kanapki

Compare:

• szybka kanapka = a quick sandwich
• szybkie kanapki = quick sandwiches
• szybki obiad = a quick lunch

English speakers often have to get used to the fact that Polish adjectives change form much more often than English ones.

They are both in the present tense, but in Polish the present tense often covers both:

• I cut / I make
• I am cutting / I am making

So:

• kroję can mean I cut or I am cutting
• robię can mean I make or I am making

The exact English translation depends on context. In this sentence, the idea is clearly a sequence of actions happening now or habitually:

• I cut sausage and bread on a board, and then I make quick sandwiches.

Yes, Polish word order is more flexible than English word order because the endings show grammatical relationships.

The given sentence is perfectly natural:

• Kiełbasę i chleb kroję na desce, a potem robię szybkie kanapki.

But you could also say:

• Kroję kiełbasę i chleb na desce, a potem robię szybkie kanapki.

The difference is mostly about focus or style:

• starting with Kiełbasę i chleb puts more attention on what is being cut
• starting with Kroję is a more neutral verb-first statement

Not every rearrangement sounds equally natural, but Polish definitely allows more freedom than English.

A few useful points:

• kieł in kiełbasę starts with a sound like kyeu or kyew, with ł pronounced like English w
• ę is a nasal vowel, but at the end of a word it is often pronounced less strongly than learners expect
• robię also ends in ę, again with a nasal quality

Rough learner-friendly approximations:

• kiełbasę ≈ kyew-BA-seh(n)
• robię ≈ RO-byeh(n)

A few key letters from this sentence:

• ł = like English w
• ę = nasal vowel
• ch = like a strong h
• ie often gives a ye-like sound after a consonant

These are only approximations, but they help as a starting point.