Uważaj na palec, kiedy kroisz cebulę ostrym nożem.

Uważaj is the imperative form of uważać, addressed to one person. It means be careful, watch out, or pay attention.

• uważaj! = be careful!
• to more than one person: uważajcie!

In this sentence, it gives a warning.

Because uważać na + accusative is a common pattern meaning to watch out for or to be careful about something.

So:

• uważać na palec = to watch out for your finger

This is just how the verb is normally used in this meaning. It is worth learning as a set phrase.

After na in this sentence, Polish uses the accusative case.

The noun palec is a masculine inanimate noun, and for masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: palec
• accusative: palec

That is why you see na palec.

Compare this with a masculine animate noun:

• pies → widzę psa

There the accusative changes, but palec does not.

Yes, it can be natural if the speaker is thinking about one specific finger that might get cut.

However, in real life, many speakers might also say:

• Uważaj na palce = Watch your fingers

The plural can sound more general. The singular is still understandable and possible.

Kroisz is the 2nd person singular present tense form of kroić.

So:

• ja kroję = I cut / I am cutting
• ty kroisz = you cut / you are cutting

Because Polish verb endings already show the subject, the word ty is usually omitted. So kroisz by itself already means you are cutting or you cut.

Polish often leaves out subject pronouns when they are obvious from the verb ending.

Here both verbs already show that the sentence is addressed to one person:

• uważaj = imperative to one person
• kroisz = you cut / are cutting

So adding ty would usually be unnecessary unless you wanted special emphasis.

Because cebula is the direct object of kroisz, so it has to be in the accusative case.

Cebula is a feminine noun, and many feminine nouns ending in -a change to -ę in the accusative singular:

• nominative: cebula
• accusative: cebulę

So:

• kroisz cebulę = you are cutting an onion

Because Polish often uses the instrumental case to express the tool or instrument used to do something.

Here the tool is a sharp knife, so both words go into the instrumental:

• ostry nóż = a sharp knife
• ostrym nożem = with a sharp knife

So:

• kroić ostrym nożem = to cut with a sharp knife

That is the instrumental singular form of nóż.

The changes are:

• nóż → nożem

Two things happen here:

1. the noun gets the instrumental ending -em
2. ó changes to o, which is a common alternation in Polish word forms

You will see similar changes in other forms too:

• nóż
• noża
• nożem

Adjectives in Polish must agree with the noun they describe in case, number, and gender.

Since nożem is instrumental singular masculine, the adjective must also be instrumental singular masculine:

• ostry nóż = nominative
• ostrym nożem = instrumental

So ostrym matches nożem.

Kiedy means when here.

It introduces a subordinate clause:

• kiedy kroisz cebulę ostrym nożem = when you cut / when you are cutting an onion with a sharp knife

Depending on context, kiedy can sometimes feel like when or whenever. In this sentence, it has a general warning sense, so English could understand it almost as whenever.

Because in Polish, subordinate clauses are normally separated by a comma.

Since kiedy kroisz cebulę ostrym nożem is a subordinate clause, a comma is expected:

• Uważaj na palec, kiedy kroisz cebulę ostrym nożem.

This is standard Polish punctuation.

No. Polish word order is fairly flexible, although some versions sound more natural than others.

This sentence could also be written as:

• Kiedy kroisz cebulę ostrym nożem, uważaj na palec.

That means the same thing. The main difference is emphasis and rhythm, not basic meaning.

The verb kroić is imperfective, and that fits this sentence well because the warning is about an ongoing or general repeated action.

• kroić = to be cutting / to cut in general
• perfective forms would suggest completion or a single finished action

A warning like this is naturally about the activity itself, so the imperfective is the usual choice.

Yes, it is understandable and basically natural. But depending on context, some speakers might more often say:

• Uważaj na palce, kiedy kroisz cebulę ostrym nożem.
• Uważaj na palce przy krojeniu cebuli ostrym nożem.

The given sentence is fine, especially if the speaker has one finger in mind. The plural palce just sounds a bit more general as a warning.