Muszę pokroić cebulę do zupy.

Muszę means I must, I have to, or I need to.

It is the 1st person singular form of the verb musieć = to have to / must.

So:

• muszę = I have to
• musisz = you have to
• musi = he/she/it has to

In this sentence, Muszę pokroić cebulę do zupy means that the speaker feels some necessity or obligation to do it.

In Polish, subject pronouns are often left out if the verb form already makes the subject clear.

• muszę already means I have to
• so ja is not necessary

You could say:

• Ja muszę pokroić cebulę do zupy

but that usually adds emphasis, something like:

• I have to cut the onion for the soup

So the version without ja is the most natural neutral sentence.

Polish does not have articles like English a, an, and the.

So cebulę can mean:

• an onion
• the onion

The exact meaning depends on context.

In a cooking sentence like this, English might say either:

• I need to chop an onion for the soup
• I need to chop the onion for the soup

Polish simply says cebulę and lets the context decide.

This is about aspect, which is very important in Polish verbs.

• kroić = imperfective: to cut, to be cutting, to cut in a general/ongoing way
• pokroić = perfective: to cut up, to finish cutting, to cut into pieces/slices

In this sentence, muszę pokroić cebulę suggests a completed result: the onion needs to get cut up.

So it is closer to:

• I need to cut up the onion

If you said Muszę kroić cebulę, it would sound more like:

• I have to be cutting the onion
• I have to cut onion(s), with more focus on the activity/process rather than the finished result

Pokroić is a fairly general cooking verb meaning to cut up.

Depending on context, English might translate it as:

• cut up
• chop
• slice

The prefix po- often gives the idea of doing the action to completion or into pieces.

So pokroić cebulę usually means something like:

• cut the onion up
• chop the onion
• slice the onion

If you want to be more specific in Polish, there are other verbs too, for example:

• posiekać cebulę = chop the onion finely
• pokroić w plasterki = slice into slices
• pokroić w kostkę = dice

Because cebulę is in the accusative case.

The dictionary form is:

• cebula = onion

But here the onion is the direct object of the verb pokroić: you are cutting the onion.

For most feminine nouns ending in -a, the accusative singular changes to -ę:

• cebula → cebulę
• zupa → zupę
• sałata → sałatę

So:

• cebula = nominative
• cebulę = accusative

Here do zupy means for the soup or literally something like for/into the soup.

In cooking Polish, do + genitive is very commonly used to mean that something is intended to go into a dish.

So:

• cebula do zupy = onion for the soup
• ziemniaki do obiadu = potatoes for dinner
• sos do makaronu = sauce for pasta

In this sentence, do zupy tells you the purpose of cutting the onion: it is being cut for the soup.

Because the preposition do requires the genitive case.

The base form is:

• zupa = soup

After do, it becomes:

• do zupy = for the soup / to the soup / into the soup

So this is another case change:

• zupa → zupy

This is a very common pattern in Polish:

• do domu = to the house/home
• do szkoły = to school
• do kawy = for the coffee

Because do and dla mean different kinds of for.

• dla usually means for someone’s benefit
  • prezent dla mamy = a present for mom
• do often means for use in / intended for / to go into
  • warzywa do zupy = vegetables for soup

So cebula do zupy means the onion is meant to be used in the soup.

If you said dla zupy, it would sound unnatural here.

Yes. Polish word order is more flexible than English word order.

The neutral version is:

• Muszę pokroić cebulę do zupy.

But you could also say:

• Cebulę muszę pokroić do zupy.
  Emphasis on the onion
• Do zupy muszę pokroić cebulę.
  Emphasis on for the soup

Even though the order can change, the case endings help show what each word is doing.

Still, the original sentence is the most natural plain, neutral version.

A rough English-style guide is:

• Muszę ≈ MOO-sheh
• pokroić ≈ po-KROYCH or po-KROY-itch
• cebulę ≈ tseh-BOO-leh
• do zupy ≈ do ZOO-pih

A few useful sound notes:

• sz sounds like English sh
• ce sounds like tse
• ć is a soft sound somewhat like a very soft ch
• ę is a nasal vowel, but at the end of a word it is often pronounced only weakly nasal in normal speech

Stress in Polish is usually on the second-to-last syllable:

• MU-szę
• po-KRO-ić
• ce-BU-lę
• ZU-py

Usually yes, cebulę is singular, so it most naturally means one onion.

But just as in English cooking language, context matters. Sometimes a singular ingredient noun can be understood a bit more generally. Still, the normal reading here is:

• I need to chop an onion / the onion for the soup

If you meant more than one onion, you would normally use a plural form, for example:

• Muszę pokroić cebule do zupy = I need to cut up onions for the soup

Yes. Good natural translations include:

• I have to chop the onion for the soup.
• I need to chop an onion for the soup.
• I need to cut up the onion for the soup.

The best English version depends on context, especially because Polish does not mark a/the.

If you want to keep the perfective feel of pokroić, cut up or chop up can work very well.