Knjige su vraćene u petak.

Knjige is the nominative plural of knjiga (book).

In this sentence, knjige is the grammatical subject of the sentence, so nominative is used. Even though in English the books were returned feels like something happened to the books, Croatian treats books as the subject in this passive-style construction.

• singular: knjiga = book
• plural nominative: knjige = books

So Knjige su vraćene literally corresponds to The books were returned.

Su vraćene is a passive/resultative-style construction meaning were returned.

It is made of:

• su = are (3rd person plural of biti, to be)
• vraćene = a passive past participle-like form meaning returned

Together, they give the sense were returned.

So:

• Knjige su vraćene = The books were returned

This is a very common way in Croatian to express what English often expresses with to be + past participle.

Because vraćene must agree with knjige.

In Croatian, this participle agrees with the noun in:

• gender
• number
• often effectively case in these constructions

Here, knjige is:

• plural
• feminine (because knjiga is feminine)

So the form is vraćene, the feminine plural form.

Compare:

• Knjiga je vraćena. = The book was returned.
• Knjige su vraćene. = The books were returned.

Because the subject knjige is plural.

Croatian verbs agree with the subject in person and number. Since knjige means books, the auxiliary verb must be plural:

• je = is / used with singular
• su = are / used with plural

So:

• Knjiga je vraćena. = singular
• Knjige su vraćene. = plural

U petak means on Friday.

Croatian often uses u + accusative for days in this kind of expression, where English uses on:

• u ponedjeljak = on Monday
• u utorak = on Tuesday
• u petak = on Friday

So although u often means in or into, here the whole phrase u petak is just the standard way to say on Friday.

Because after u in this time expression, Croatian uses the accusative, and for petak the accusative singular is also petak.

So:

• nominative: petak
• accusative: petak

Since those two forms are identical for this noun, the form does not change visibly.

That is why you get u petak.

Yes, it functions like a passive sentence in English.

Knjige su vraćene u petak. corresponds to:

• The books were returned on Friday.

Croatian often forms this kind of passive meaning with:

• a form of biti (to be)
• plus a participle like vraćen, vraćena, vraćene, etc.

So it is natural to think of this sentence as passive from an English-speaking learner’s point of view.

Yes. Croatian could express the same basic idea actively if the doer is mentioned.

For example:

• Netko je vratio knjige u petak. = Someone returned the books on Friday.
• Knjižničar je vratio knjige u petak. = The librarian returned the books on Friday.

The passive-style sentence Knjige su vraćene u petak is useful when the focus is on the books or when the person who returned them is unknown or unimportant.

The form vraćene comes from the verb vratiti or is related to the verb pair vraćati / vratiti, meaning to return, to give back, or to bring back, depending on context.

Very roughly:

• vraćati = imperfective, ongoing/repeated
• vratiti = perfective, completed

Since this sentence refers to a completed action, the perfective idea fits well: the books were returned.

Because Croatian has no articles.

There is no direct equivalent of the or a/an in normal Croatian grammar. So knjige can mean:

• books
• the books

The exact meaning depends on context. In this sentence, English would usually translate it as the books because the sentence sounds like it refers to a specific set of books.

Yes, Croatian word order is more flexible than English word order.

The neutral order here is:

• Knjige su vraćene u petak.

But other orders are possible depending on emphasis, for example:

• U petak su vraćene knjige. = emphasizes on Friday
• Vraćene su knjige u petak. = possible, but less neutral in many contexts

The most natural choice depends on what the speaker wants to highlight. The original sentence is a straightforward, neutral version.

In form, Croatian does not map neatly onto English tense names, so the best natural translation depends on context. But in normal English, Knjige su vraćene u petak is most naturally:

• The books were returned on Friday.

Because u petak gives a finished time reference, English usually prefers the simple past, not the present perfect.

So even if the Croatian structure looks a bit like have been returned, the normal English translation with on Friday is were returned.

It would be:

• Knjiga je vraćena u petak.

Changes:

• knjige → knjiga (plural to singular)
• su → je (plural auxiliary to singular auxiliary)
• vraćene → vraćena (feminine plural to feminine singular)

This shows how the auxiliary and participle agree with the noun.