Learn Hardcore Croatian: Knjiga koju čitam svake večeri je dobra. - The book that I read every evening is good.

Because koju is the accusative feminine singular form of the relative pronoun koji. In the relative clause, the pronoun refers to knjiga (feminine), but inside that clause it is the direct object of čitam (I read), so it must be in the accusative: koju. Koja is nominative and would be used if the relative pronoun were the subject of the relative clause.

What does the relative pronoun agree with, and how is its case chosen?

Agreement: koju agrees with its antecedent knjiga in gender (feminine) and number (singular).
Case: it takes the case required by its role in the relative clause. Here it’s the direct object of čitam, so accusative. Examples:
Indirect object: Knjiga kojoj se divim je stara. (dative: I admire the book)
After a preposition: Knjiga o kojoj govorim je nova. (locative after “o”)

Why is there no comma before koju?

Because the clause koju čitam svake večeri is restrictive: it specifies which book. Croatian normally does not use commas with restrictive relative clauses. With commas, it becomes non‑restrictive (extra information): Knjiga, koju čitam svake večeri, je dobra. That version implies there’s only one relevant book, and “which I read every evening” is parenthetical.

What does svake večeri mean exactly, and why that form?

It means “every evening.” It’s the genitive singular of večer with the adjective svaki declined to match: svake (fem. gen. sg.) večeri (gen. sg.). Croatian often uses the genitive to express habitual time: svakog dana, svake godine, svake večeri.

Can I also say svaku večer instead of svake večeri?

Yes. Both are standard:

svake večeri (genitive) — common and slightly more formal/neutral.
svaku večer (accusative functioning adverbially) — very common in speech. Avoid mixing gender/case: not svaki večer.

Why does večeri end in -i here? Isn’t that plural?

In svake večeri, večeri is genitive singular of večer (a feminine noun). It happens to be homonymous with nominative plural (also večeri). Context shows it’s genitive singular here.

What is the role of je, and why is it placed there?

Je is the clitic form of “to be” (3rd person singular: “is”). Croatian clitics tend to appear in second position within their clause, after the first stressed constituent. The first constituent here is the whole noun phrase Knjiga koju čitam svake večeri, so je comes right after it: … je dobra.

Can I move je? For example, is Dobra je knjiga koju čitam svake večeri okay?

Yes. If you front dobra for emphasis, je still seeks second position in that clause: Dobra je knjiga koju čitam svake večeri. The meaning is the same, but the focus shifts to “good.”

Why is it dobra and not dobar or dobro?

Predicate adjectives agree with the subject in gender and number. Knjiga is feminine singular, so the adjective is dobra (feminine singular nominative). Masculine would be dobar, neuter dobro.

Why not say Knjiga je dobra koju čitam svake večeri?

In Croatian, the relative clause must stay adjacent to its head noun; it can’t be stranded after the predicate like in that sentence. Keep the head and its relative clause together: Knjiga koju čitam svake večeri je dobra.

Can I drop the relative pronoun, like “The book I read every evening is good” in English?

No. Croatian generally requires the relative pronoun. You need koju: Knjiga koju čitam svake večeri je dobra.

Is it okay to say Knjiga koju je/ju čitam svake večeri?

No. Don’t use a resumptive object clitic (je/ju) when the relative pronoun already stands for that object. Use only the relative pronoun: Knjiga koju čitam svake večeri…

What does čitam tell us? Do I need to say ja?

Čitam is present, 1st person singular (“I read”). Croatian is pro‑drop: the subject pronoun ja is usually omitted because the verb ending shows the person. You could say Ja čitam, but it’s only needed for emphasis or contrast.

Can I move svake večeri inside the relative clause?

Yes. Both are fine:

Knjiga koju čitam svake večeri je dobra.
Knjiga koju svake večeri čitam je dobra. The difference is only in rhythm/emphasis.

Why isn’t there an article like “the” before knjiga?

Croatian has no articles. Definiteness is inferred from context, word order, pronouns, or modifiers (e.g., ona knjiga = “that book”).

Can I use što instead of koju?

In standard Croatian, after a noun you use the relative koji/koja/koje. Što is used in other patterns (e.g., ono što, sve što) or in some colloquial/regional speech. Here, stick with koju.

What case is knjiga in here, and why isn’t it knjigu?

Knjiga is nominative singular because it’s the subject of the main clause. Knjigu (accusative) would be the form for a direct object, e.g., Čitam knjigu.

Are there natural paraphrases without a relative clause?

Yes, for example:

Two sentences: Knjiga je dobra. Svake večeri je čitam.
Using a demonstrative: Ona knjiga koju čitam svake večeri je dobra.
Changing focus: Dobra je ona knjiga koju čitam svake večeri.