Kad završim knjigu, vraćam je u knjižnicu.

Both kad and kada mean when.

• kad is shorter and very common in everyday speech
• kada is a bit fuller and can sound slightly more formal or careful

So Kad završim knjigu, vraćam je u knjižnicu. and Kada završim knjigu, vraćam je u knjižnicu. are both correct.

This is very normal in Croatian.

After kad meaning when, Croatian often uses the present tense of a perfective verb to talk about a future completed action.

So:

• završim = literally a present form
• but here it means when I finish

The verb završiti is perfective, so it refers to a completed action. That is why Croatian can use kad završim where English uses when I finish.

Because knjigu is the accusative singular form of knjiga.

Here, knjigu is the direct object of završim:

• knjiga = nominative, the basic dictionary form
• knjigu = accusative, used for the thing being finished/read/etc.

So the pattern is:

• završiti knjigu
• čitati knjigu
• vratiti knjigu

This is a good aspect question.

• vraćam is imperfective
• vratim would be perfective present
• vratit ću is the explicit future tense

In Croatian, the present tense can sometimes be used for a future or planned action, especially when the context already makes the time clear.

So Kad završim knjigu, vraćam je u knjižnicu. can mean something like When I finish the book, I’m returning it to the library / I return it to the library / I’ll return it to the library, depending on context.

If you want a more clearly one-time future result, you could also say:

• Kad završim knjigu, vratit ću je u knjižnicu.

That version is also very natural.

This is about verbal aspect, which is very important in Croatian.

• završiti = perfective → to finish, to bring something to completion
• vraćati = imperfective → to return, be returning, return repeatedly/habitually, or sometimes a planned action in context

In this sentence:

• završim focuses on the moment the book is finished
• vraćam presents the return as the next action, without focusing as strongly on completion

A learner should notice that Croatian often mixes perfective and imperfective very naturally depending on how the speaker views each action.

Je here means it.

It refers back to knjigu.

So:

• knjigu = the book
• je = it

Croatian often uses a short object pronoun instead of repeating the noun:

• Kad završim knjigu, vraćam je u knjižnicu.
• literally: When I finish the book, I return it to the library.

Because je is an unstressed clitic pronoun. Croatian clitics usually go in a special position near the beginning of the clause, often after the first stressed word.

In the main clause here, the first word is vraćam, so the clitic je comes right after it:

• vraćam je

That is the normal order.

Croatian word order is flexible, but clitics like je have special placement rules, so learners have to get used to them.

Because u can take different cases depending on meaning:

• u + accusative = movement into/to
• u + locative = location in

Here there is movement toward the library, so Croatian uses the accusative:

• u knjižnicu = to the library

Compare:

• Vraćam je u knjižnicu. = I return it to the library
• Ona je u knjižnici. = She is in the library

This is a very important pattern in Croatian.

Because Croatian usually does not need subject pronouns when the verb ending already shows the subject.

• završim = I finish
• vraćam = I return

So ja is not necessary.

You can add ja for emphasis:

• Kad ja završim knjigu, vraćam je u knjižnicu.

But without emphasis, leaving it out is more natural.

Yes, in standard Croatian the comma is expected here.

Kad završim knjigu is a subordinate clause, and it is separated from the main clause:

• Kad završim knjigu, vraćam je u knjižnicu.

Croatian uses commas with subordinate clauses more regularly than English learners sometimes expect.

Either is possible, depending on context.

Croatian has no articles like a or the, so knjigu by itself does not tell you definiteness.

It can mean:

• a book
• the book

The context tells you which meaning is intended.

That is why Croatian learners of English have to learn articles, and English speakers learning Croatian have to get used to not seeing them.

Yes. Knjižnica is the standard Croatian word for library.

A useful contrast is:

• knjižnica = library
• knjižara = bookshop / bookstore

English speakers sometimes mix those up because both words are related to knjiga meaning book.

Yes. Croatian word order is flexible, although some versions are more neutral than others.

For example:

• Kad završim knjigu, vratit ću je u knjižnicu.
  More explicitly future.

• Kada završim knjigu, vraćam je u knjižnicu.
  Same meaning, just with kada.

The original sentence is perfectly natural, but Croatian often allows several correct versions with slightly different style or nuance.