Knjiga je već prodana.

Because knjiga is the subject of the sentence, so it is in the nominative case.

• knjiga = nominative singular, the book
• knjigu = accusative singular, used when book is the direct object

In Knjiga je već prodana, the book is not the thing doing the action in an active sense, but grammatically it is still the subject of this passive sentence.

Compare:

• Prodajem knjigu. = I am selling the book.
  Here knjigu is the object.
• Knjiga je prodana. = The book has been sold / The book is sold.
  Here knjiga is the subject.

Je is the 3rd person singular present form of biti (to be).

Here it works as an auxiliary verb in a passive construction:

• knjiga = the book
• je = is / has been
• prodana = sold

So je prodana together means something like:

• is sold
• has been sold

The exact English translation depends on context, but in Croatian this structure is very normal for a passive meaning.

Because prodana is not the infinitive. It is a passive participle built from the verb prodati (to sell).

In this sentence, the participle must agree with knjiga in gender and number:

• knjiga is feminine singular
• so the participle is prodana = sold (feminine singular)

Other forms would be:

• prodan for a masculine singular noun
• prodano for a neuter singular noun
• prodane / prodani / prodana for plural, depending on gender pattern

For example:

• Auto je prodan. = The car has been sold.
• Pismo je prodano. = The letter has been sold.
• Knjige su prodane. = The books have been sold.

Yes. Knjiga je već prodana is a passive construction.

The active version would be something like:

• Netko je već prodao knjigu. = Someone has already sold the book.

The passive version focuses on the book, not on who sold it:

• Knjiga je već prodana.

This is very common in Croatian when the seller is unknown, unimportant, or obvious from context.

Već usually means already.

So it adds the idea that the action happened before now, often earlier than expected or before some other event.

• Knjiga je prodana. = The book has been sold.
• Knjiga je već prodana. = The book has already been sold.

Depending on context, već can sometimes also add a nuance like by now or as it turns out, already, but already is the basic meaning here.

Yes, Croatian word order is fairly flexible, but not completely free.

In this sentence, je is a clitic, and clitics usually go in the second position in the clause. That is why je comes very early:

• Knjiga je već prodana.

This is the most neutral order.

You may also hear or see other orders for emphasis, for example:

• Knjiga je prodana već.
• Već je knjiga prodana.
• Knjiga כבר je prodana. ← this one is wrong because je should not be separated in unnatural ways

The standard, natural version for most situations is:

• Knjiga je već prodana.

So the main thing to remember is: clitics like je tend to come early, usually in second position.

It can correspond to either in English, depending on context.

Croatian je + passive participle often covers meanings that English splits into:

• is sold
• has been sold

In this sentence, with već, the most natural English idea is usually:

• The book has already been sold.

That is because already strongly suggests a completed action with present relevance.

So although the Croatian form uses present je, the overall meaning is often closer to English has been sold.

The correct Croatian word would be prodata, not כבר prodata. The form prodata does exist and many speakers use it, but prodana is the usual standard Croatian form.

So:

• Knjiga je već prodana. = standard Croatian
• Knjiga je već prodata. = also understandable, but more typical of some regional usage and more associated with Serbian or non-standard usage in Croatian contexts

If you are learning standard Croatian, prodana is the safer choice.

You make it negative by changing je to nije:

• Knjiga nije još prodana. = The book has not been sold yet.
• Knjiga nije već prodana. is usually not the natural way to say it.

In practice, Croatian often uses još (yet/still) in negative sentences rather than već:

• Knjiga je već prodana. = The book has already been sold.
• Knjiga još nije prodana. = The book has not been sold yet.

That pair is very useful to memorize.

Yes, and it is similar in meaning, but it is not exactly the same structure.

• Knjiga je već prodana. = passive; focuses on the result that the book is sold
• Knjiga se već prodala. = reflexive/intransitive-style expression; often sounds a bit more like the book sold itself / the book got sold, especially in everyday speech

Both can be natural, but je prodana is the clearer passive pattern and is especially useful for learners because it shows the agreement clearly.

If you want the safest standard sentence matching has already been sold, use:

• Knjiga je već prodana.