Bibliotekarka poprosiła, żebym oddała książkę w piątek, bo inny czytelnik już na nią czeka.

Because bibliotekarka is a feminine noun meaning a female librarian, and in the past tense Polish verbs agree with the gender of the subject.

• bibliotekarka → feminine
• poprosiła → feminine past tense form of poprosić

If the subject were masculine, you would get poprosił instead.

Żebym is a very common Polish form meaning something like that I should, for me to, or so that I would, depending on context.

It is made from:

• żeby
  • -m

That -m is an old clitic form meaning I.

So:

• żebym oddała książkę = that I return the book / for me to return the book

After verbs like prosić (to ask/request), Polish very often uses żeby-clauses:

• Poprosiła, żebym... = She asked me to...

This is one of the most important things to notice.

After żeby, Polish usually uses forms that look like past tense forms, but here they do not mean past time. They are part of a special pattern used after żeby.

So in:

• żebym oddała książkę w piątek

oddała does not mean I returned. It means that I return / that I should return.

This is normal Polish grammar. The same pattern appears very often:

• Chcę, żeby przyszedł. = I want him to come.
• Prosiła, żebym zadzwoniła. = She asked me to call.

Because of oddała.

In this kind of żeby clause, the verb still shows the speaker’s gender in the singular:

• żebym oddał → if the speaker is male
• żebym oddała → if the speaker is female

So this sentence tells us that the person speaking is female.

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

• nominative: książka
• accusative: książkę

Since the action is being done to the book, Polish uses the accusative:

• oddać książkę = to return the book

This is very common with transitive verbs.

When Polish talks about on a day of the week, it normally uses:

• w + accusative

So:

• w piątek = on Friday
• w poniedziałek = on Monday
• w czwartek = on Thursday

This is just the standard pattern for days of the week in this kind of time expression.

Because the verb czekać requires the preposition na when it means to wait for.

So the pattern is:

• czekać na kogo? na co?
  • accusative

Examples:

• czekać na autobus
• czekać na koleżankę
• czekać na nią

So na nią is not optional here; it is required by the verb.

Because after a preposition, Polish uses a different pronoun form.

Here the preposition is na, so you need the accusative form of ona:

• nominative: ona
• accusative after na: nią

So:

• czeka na nią = is waiting for her/it

You cannot use ona after na in this sentence.

Już usually means already, but in context it can also suggest by now or as it is.

Here it adds the idea that the other reader is not just going to wait at some future point — they are already waiting now.

So it makes the request sound more urgent or more justified:

• inny czytelnik już na nią czeka
  = another reader is already waiting for it

Because inny czytelnik is the subject of the clause inny czytelnik już na nią czeka, so it stays in the nominative case.

• inny czytelnik = nominative singular
• subject of czeka

If it were an object, the case might change, but here this person is the one doing the waiting.

Yes, in many contexts it could.

• bo = very common, everyday because
• ponieważ = a bit more formal or neutral in style

So:

• ..., bo inny czytelnik już na nią czeka.
• ..., ponieważ inny czytelnik już na nią czeka.

Both are correct. In ordinary speech, bo is extremely common.

Polish uses commas before words like żeby and bo when they introduce subordinate clauses.

So here:

• Bibliotekarka poprosiła, żebym oddała książkę w piątek, bo inny czytelnik już na nią czeka.

There is:

1. a comma before żebym, because it introduces what she asked
2. a comma before bo, because it introduces the reason

This is standard Polish punctuation.

Yes, Polish word order is relatively flexible, although some versions sound more natural than others.

For example, these are possible:

• Bibliotekarka poprosiła, żebym oddała książkę w piątek, bo inny czytelnik już na nią czeka.
• Bibliotekarka poprosiła, żebym w piątek oddała książkę, bo inny czytelnik już na nią czeka.
• ..., bo już inny czytelnik na nią czeka.

The basic meaning stays the same, but the emphasis changes slightly. The original sentence is natural and neutral.

The basic verb is oddać, which is perfective.

Here it means to give back / return. In a library context, oddać książkę is the normal way to say return a book.

Related pair:

• oddawać = imperfective
• oddać = perfective

In this sentence, oddać fits well because it refers to completing the act of returning the book.