Nie wiem, czego ona chce.

Czego is the genitive form of co. In this sentence, that is the normal standard choice after chcieć in an indirect question like czego ona chce.

English uses the same word, what, no matter what role it has. Polish changes the form of the pronoun depending on case, so what can become co, czego, and other forms depending on the grammar.

You may sometimes hear co ona chce in casual speech, but czego ona chce is the safer and more standard form for a learner.

It is genitive.

The basic form is co. When it changes to genitive, it becomes czego.

A useful comparison is:

• Co to jest? = What is this?
• Czego ona chce? = What does she want?

So one important thing to notice here is that Polish question words decline just like nouns and pronouns do.

Because chce is the conjugated form, while chcieć is the dictionary form.

Here the subject is ona = she, so you need 3rd person singular:

• ja chcę = I want
• ty chcesz = you want
• ona chce = she wants
• my chcemy = we want
• wy chcecie = you want
• oni/one chcą = they want

So ona chce means she wants.

Not always. Polish often leaves out subject pronouns when the verb already makes the subject clear.

So you can also say:

Nie wiem, czego chce.

That is perfectly natural if it is already obvious who she is.

Keeping ona can add:

• clarity, if several people are being discussed
• emphasis, especially if you mean she, not someone else

So the version with ona is correct, but the pronoun is not always required.

Because czego ona chce is a subordinate clause: more specifically, an indirect question embedded inside the larger sentence.

Polish normally puts a comma before such clauses:

• Nie wiem, czego ona chce.
• Pytam, gdzie on jest.
• Nie pamiętam, kiedy to było.

So the comma is not optional here in standard writing.

Because the whole sentence is a statement, not a direct question.

The speaker is not directly asking what does she want? They are stating I don't know what she wants.

Compare:

• Czego ona chce? = direct question
• Nie wiem, czego ona chce. = statement containing an indirect question

That is why the sentence ends with a period.

Yes. Polish word order is fairly flexible, but different orders change the emphasis.

The most neutral order here is:

Nie wiem, czego ona chce.

Other possibilities are possible, for example:

• Nie wiem, czego chce ona. — puts more stress on ona
• Czego ona chce, nie wiem. — more marked, stylistic, or contrastive

So the original version is a good neutral model to learn first.

For a learner, it is best to treat Nie wiem, czego ona chce as the correct standard form.

You may hear co ona chce in colloquial speech, and native speakers do use it in some contexts. But in careful, standard Polish, czego is preferred here.

So the practical rule is:

• recognize that co ona chce may appear in real life
• use czego ona chce yourself if you want the standard form

It is an indirect question embedded inside a bigger sentence.

The main clause is:

Nie wiem = I don't know

The embedded clause is:

czego ona chce = what she wants

So the whole sentence is built like this:

• main clause: I don’t know
• embedded question: what she wants

This is very common in Polish, just as in English:

• Wiem, gdzie on mieszka.
• Nie pamiętam, kiedy przyjdzie.
• Powiedz mi, czego potrzebujesz.