Bibliotekarka zostawiła mi zakładkę w książce, bo wiedziała, że lubię wracać do tego samego akapitu.

Bibliotekarka means female librarian. In Polish, many profession nouns show gender:

• bibliotekarz = male librarian
• bibliotekarka = female librarian

Because the sentence is about a woman, the feminine form bibliotekarka is used.

In the past tense, Polish verbs agree with the gender of the subject.

Since bibliotekarka is feminine singular, the past-tense verbs are also feminine singular:

• zostawił = he left
• zostawiła = she left
• wiedział = he knew
• wiedziała = she knew

So zostawiła and wiedziała both match bibliotekarka.

Both mi and mnie can mean to me or me, but mi is the unstressed short form and is very common in normal sentences.

Here, zostawiła mi zakładkę means she left me a bookmark.

Compare:

• Dała mi książkę. = She gave me a book.
• Mnie dała książkę, nie jemu. = She gave the book to me, not to him.

So mnie is more likely when you want emphasis or contrast. In this sentence, plain mi is the natural choice.

Because zakładkę is the accusative singular form, and it is the direct object of zostawiła.

Dictionary form:

• zakładka = bookmark

Accusative singular:

• zakładkę

So:

• zostawiła zakładkę = she left a bookmark

This is very common with feminine nouns ending in -a:

• książka → książkę
• kartka → kartkę
• zakładka → zakładkę

The preposition w often means in and usually requires the locative case when talking about location.

So:

• książka = book
• w książce = in the book

The noun changes to the locative singular:

• książka → książce

This is why the sentence has w książce, not w książka.

Bo means because. It is very common in everyday Polish and sounds natural in speech.

So:

• ..., bo wiedziała... = ..., because she knew...

Yes, you could also use ponieważ, which also means because, but it usually sounds a bit more formal or more written.

• bo = common, conversational
• ponieważ = slightly more formal

Both would be grammatically correct here.

Że means that and introduces a subordinate clause.

So:

• wiedziała, że... = she knew that...

In English, that is often optional:

• She knew (that) I like...

In Polish, że is normally expressed here, so wiedziała, że... is the standard structure.

This is a very common thing learners notice.

Polish often keeps the present tense in the subordinate clause if the statement is still true at the time of speaking.

So:

• wiedziała, że lubię... = she knew that I like...

This suggests: she knew this, and it is still true now that I like doing it.

If you said:

• wiedziała, że lubiłem...

that would mean she knew that I liked..., with the liking presented more as a past fact.

So the present tense after a past verb is normal in Polish when the fact is still current.

Wracać means to return / to keep coming back and is an imperfective verb.

With do, it means to return to something:

• wracać do książki = to return to a book
• wracać do tematu = to return to a topic
• wracać do akapitu = to return to a paragraph

In this sentence, lubię wracać do tego samego akapitu means that the speaker likes going back to that paragraph, probably more than once. That repeated or ongoing sense fits the imperfective verb wracać very well.

Because the preposition do requires the genitive case.

The phrase is built like this:

• ten sam akapit = the same paragraph
  
  • ten = this/that
  • sam = same
  • akapit = paragraph

After do, all the relevant words must go into the genitive:

• ten → tego
• sam → samego
• akapit → akapitu

So:

• do tego samego akapitu = to the same paragraph

This kind of matching is normal in Polish: adjectives and determiners agree with the noun in case, number, and gender.

Yes. Sam in this phrase means same.

Compare:

• ten sam akapit = the same paragraph
• to samo miejsce = the same place
• ta sama książka = the same book

In your sentence, it becomes:

• do tego samego akapitu

because of the genitive after do.

Be careful: sam can also mean alone in other contexts, but here with ten it clearly means same.

Polish word order is more flexible than English, but some orders sound more natural than others.

Zostawiła mi zakładkę is the most neutral and natural order here:

• verb
• indirect object (mi = to me)
• direct object (zakładkę = bookmark)

You can move things around for emphasis, but it changes the feel:

• Zostawiła zakładkę mi — possible only in a very marked, contrastive context, and usually sounds unnatural
• Mi zostawiła zakładkę — emphasizes to me
• Zakładkę zostawiła mi — emphasizes bookmark

So the original order is the best default.

Yes, you could say other things, but they would mean something slightly different.

In the original sentence:

• w książce = in the book
• do tego samego akapitu = to the same paragraph

The speaker is not specifying which book with tej samej or tej; they are just saying it happened in a book or in the book understood from context. But they are specifying the paragraph as the same paragraph.

So the sentence focuses on the repeated return to one particular paragraph, not on identifying the book.

In this context, yes, zakładka clearly means bookmark.

The noun zakładka can have other meanings in other contexts, for example a tab in a browser or computer interface. But with:

• zostawiła mi zakładkę w książce

the meaning is unmistakably bookmark left in a book.

You would change both the noun and the past-tense verb forms:

• Bibliotekarz zostawił mi zakładkę w książce, bo wiedział, że lubię wracać do tego samego akapitu.

Changes:

• bibliotekarka → bibliotekarz
• zostawiła → zostawił
• wiedziała → wiedział

Everything else stays the same.

This sentence is a nice example of several cases at once:

• Bibliotekarka — nominative
  the subject

• mi — dative
  to me

• zakładkę — accusative
  the direct object

• w książce — locative after w
  in the book

• do tego samego akapitu — genitive after do
  to the same paragraph

So one sentence gives you a compact review of Polish case usage.