Nie chcę przetrzymać książki.

Because chcę is the 1st person singular form of chcieć in the present tense: I want.

• chcę = I want
• chce = he/she/it wants

So Nie chcę... means I do not want.... The ending -ę is what marks I here.

Polish often leaves out subject pronouns when the verb ending already shows who the subject is. Since chcę already means I want, adding ja is not necessary.

So:

• Nie chcę przetrzymać książki. = normal, neutral
• Ja nie chcę przetrzymać książki. = I don’t want to keep the book too long, with extra emphasis or contrast

This is very common in Polish.

After chcieć meaning to want, Polish normally uses another verb in the infinitive, just like English uses to plus the base verb:

• chcę czytać = I want to read
• chcę iść = I want to go
• chcę przetrzymać = I want to keep too long / overkeep

So Nie chcę przetrzymać książki literally has the structure I don’t want to overkeep the book.

This is mainly about aspect and also about the meaning added by the prefix prze-.

• trzymać = to hold / to keep
• przetrzymać = to keep too long / keep beyond the proper time, and it is perfective
• przetrzymywać = the imperfective partner of przetrzymać

In this sentence, przetrzymać suggests a completed result: keeping the book longer than you should.

Very roughly:

• Nie chcę trzymać książki. = I don’t want to keep/hold the book
• Nie chcę przetrzymać książki. = I don’t want to keep the book too long / overdue
• Nie chcę przetrzymywać książki. = I don’t want to be keeping the book too long / I don’t want to keep it longer than allowed

In many real situations, especially with a library or borrowed book, przetrzymać książkę strongly suggests keeping it past the due date or longer than agreed.

Because Polish often uses the genitive instead of the accusative for a direct object in a negative sentence. This is called the genitive of negation.

Compare:

• Chcę przetrzymać książkę. = I want to keep the book too long
• Nie chcę przetrzymać książki. = I don’t want to keep the book too long

Here:

• książka = dictionary form
• książkę = accusative singular
• książki = genitive singular

A learner may notice that książki can also mean books in other contexts, but here it is singular, not plural. The negation tells you that this is the genitive singular form.

Grammatically, nie is attached to chcę, but in meaning it negates the whole predicate want to keep the book too long.

So the sentence means that the speaker does not want the action of overkeeping the book to happen.

Polish normally expresses this idea with:

• Nie chcę przetrzymać książki.

A version like Chcę nie przetrzymać książki sounds awkward and is not the normal way to say it.

Yes. Polish word order is flexible because endings carry a lot of the grammar. But changing the order usually changes the focus or emphasis, not the basic meaning.

Neutral order:

• Nie chcę przetrzymać książki.

Other possible orders:

• Książki nie chcę przetrzymać. = emphasis on the book
• Nie chcę książki przetrzymać. = also possible, but more marked

For a learner, the original order is the safest and most natural neutral version.

It usually does not just mean physically holding a book in your hands. It more often means keeping possession of a book longer than you should.

Typical contexts:

• a borrowed book
• a library book
• a book lent by a friend

So the idea is often something like:

• keeping it past the deadline
• returning it late
• keeping it longer than agreed

That is why przetrzymać is a stronger and more specific choice than simple trzymać.

With ordinary verbs, yes: nie is normally written separately.

So:

• nie chcę
• nie mogę
• nie wiem

That is why the sentence has Nie chcę, not one combined word.

This is different from many adjectives or adverbs, where nie is often written together, for example niedobry or niedługo. But with verbs, separate spelling is the standard rule.