Przetrzymałem książkę i zalegam ze zwrotem; naliczą opłatę za przetrzymanie.

It comes from the verb przetrzymać, which in this context means to keep something longer than allowed or to keep it past the due date.

So in a library context, przetrzymałem książkę means that the speaker kept the book too long.

Grammatically, przetrzymałem is:

• 1st person singular
• past tense
• masculine speaker

A female speaker would say przetrzymałam.

This verb is perfective, so it presents the action as a completed whole: the speaker has already ended up in the state of having overdue-held the book.

Because książkę is the accusative singular form, and it is the direct object of przetrzymałem.

The base form is:

• książka = book

But after a transitive verb like przetrzymać, Polish uses the accusative:

• przetrzymałem książkę

This is very normal for feminine nouns ending in -a:

• nominative: książka
• accusative: książkę

Yes, przetrzymać is perfective.

Its usual imperfective partner is przetrzymywać.

Very roughly:

• przetrzymywać = to be keeping too long, to keep overdue as an ongoing/repeated situation
• przetrzymać = to keep too long, viewed as a completed fact

Examples:

• Przetrzymywałem tę książkę przez miesiąc.
  This focuses on the ongoing situation.
• Przetrzymałem tę książkę.
  This presents it as a completed fact: I ended up overdue with it.

The expression zalegać z + instrumental means to be behind with, to be overdue with, or to be in arrears with something.

So:

• zalegam ze zwrotem = I am overdue with the return / I still owe the return

This is a fairly idiomatic Polish way to talk about not having done something on time, especially in administrative or formal contexts.

It is less literally about the physical act of returning and more about the fact that the obligation is still outstanding.

Ze is just a pronunciation-friendly variant of z.

Polish often uses ze instead of z before difficult consonant clusters. Since zwrotem begins with zwr-, ze zwrotem sounds smoother than z zwrotem.

So:

• z / ze both mean with in forms like this
• here ze is chosen for ease of pronunciation

This is very common:

• z mną
• ze mną

Because after zalegać z / ze, Polish uses the instrumental case.

The noun is:

• zwrot = return

Its instrumental singular is:

• zwrotem

So:

• zalegać ze zwrotem

This is not unique to this sentence; it is part of the pattern required by the verb phrase zalegać z czymś.

Zwrot is a noun meaning return, especially in formal, institutional, or administrative language.

In a library context, zwrot książki sounds very natural and official.

Oddanie can also mean returning/giving back, but it often feels a bit more everyday and action-focused.

So the difference is roughly:

• zwrot = more formal, institutional, standard library wording
• oddanie = more general, everyday giving back

That makes zalegam ze zwrotem sound particularly suitable for things like libraries, rentals, fines, and official notices.

Naliczą is 3rd person plural from naliczyć, meaning they will charge/calculate/apply.

In Polish, the subject is often omitted when it is understood from context. Here, they means something like:

• the library staff
• the system
• the institution

So Polish often uses this kind of subjectless 3rd person plural where English might say:

• they will charge a fee
• you’ll be charged a fee

It is a very common way to refer to unnamed people or an institution.

That is because naliczyć is a perfective verb.

In Polish, perfective verbs do not have a normal present tense meaning. Their present-looking forms are used with future meaning.

So:

• naliczają = they are charging / they charge repeatedly
  imperfective, from naliczać
• naliczą = they will charge
  perfective, from naliczyć

This is one of the most important aspect patterns in Polish.

Because opłata is the direct object of naliczą, so it goes into the accusative singular.

Base form:

• opłata = fee, charge

Accusative singular:

• opłatę

So:

• naliczą opłatę = they will charge a fee

Again, this is the normal feminine -a → -ę pattern in the accusative singular.

Przetrzymanie is a verbal noun meaning something like keeping something too long, overdue retention, or overholding.

It is built from the same verb family as przetrzymać / przetrzymywać.

So:

• opłata za przetrzymanie = a fee for keeping it too long / an overdue fee

After za in this meaning of for, Polish usually takes the accusative.
But przetrzymanie is a neuter noun whose nominative and accusative singular look the same, so the form does not change visibly.

Yes, it is natural, but it sounds somewhat formal or administrative, especially because of:

• zalegam ze zwrotem
• naliczą opłatę
• za przetrzymanie

This is the sort of wording you might see in:

• library communication
• official reminders
• regulations
• customer service language

In casual speech, someone might choose simpler wording, but this sentence is perfectly natural in a library or institutional context.

The semicolon links two closely related clauses:

1. the problem: Przetrzymałem książkę i zalegam ze zwrotem
2. the consequence: naliczą opłatę za przetrzymanie

A semicolon is a stylistic choice showing a strong connection between the two parts, while still separating them more clearly than a comma would.

You could also see:

• a full stop, for a stronger break
• sometimes a comma in less careful writing

But the semicolon works well here because it neatly connects cause and consequence.