Learn Hardcore Croatian: Učitelj zna čija je bilježnica. - The teacher knows whose notebook it is.

Because čiji/čija/čije agrees with the gender and number of the noun it refers to. Bilježnica is feminine singular, so the nominative feminine form čija is used.

Masculine singular: čiji (Čiji je ruksak?)
Feminine singular: čija (Čija je bilježnica?)
Neuter singular: čije (Čije je pismo?) In other cases, the form changes: e.g., accusative feminine čiju in Čiju bilježnicu tražimo?

Why is je after čija in čija je bilježnica?

Je is a clitic (the short form of biti “to be”) and must stand in second position in its clause. The embedded question starts with čija, so je comes immediately after it: čija je bilježnica. Variants like čija bilježnica je are not standard in neutral speech.

Is this a question? Why is there a period and not a question mark?

It’s an embedded (indirect) question: Učitelj zna [čija je bilježnica]. The whole sentence is a statement about what the teacher knows, so it ends with a period. If you asked directly, you’d write: Čija je bilježnica?

Do I need a comma before čija?

No. Content clauses introduced by a question word (like čija, tko, gdje) are not preceded by a comma when they serve as the object of the verb: Učitelj zna čija je bilježnica.

Is there an “it” in Croatian like in “whose notebook it is”?

Croatian doesn’t need an explicit “it” here. The copula je links the predicate čija with the subject bilježnica. You can optionally add the demonstrative to for clarity: Učitelj zna čija je to bilježnica (“whose notebook that is”).

Could I say Učitelj zna da čija je bilježnica?

No. Da introduces declarative content clauses, not wh-clauses. Use the wh-word alone: Učitelj zna čija je bilježnica. With a declarative you’d say, e.g., Učitelj zna da je bilježnica Anina (“The teacher knows that the notebook is Ana’s”).

Why not koga je bilježnica for “whose notebook”?

Because koga is a form of tko (“who”), not “whose.” Croatian uses the possessive interrogative čiji (declined like an adjective) for “whose.” So you need čija with bilježnica: čija je bilježnica.

Which case is bilježnica in, and what’s the structure of the clause čija je bilježnica?

Bilježnica is in the nominative; it’s the subject of the copular clause. Čija functions as a predicative (agreeing with the subject). The structure is literally “whose is [the] notebook.”

Could I front the subject and say Učitelj zna bilježnica je čija?

No; that word order is not idiomatic. Keep the wh-word first and the clitic second: Učitelj zna čija je bilježnica. In a direct question, Čija je bilježnica? is standard; Bilježnica je čija? is possible only as an echo or for special emphasis.

Can I use a different phrasing to avoid čiji?

Yes. For example:

Učitelj zna kome pripada bilježnica. (“The teacher knows to whom the notebook belongs.”)
Učitelj zna tko je vlasnik bilježnice. (“The teacher knows who the owner of the notebook is.”)

What’s the difference between znati and poznavati here?

Use znati for facts and information: Učitelj zna čija je bilježnica.
Poznavati means “to be acquainted with” (people, places, works): Učitelj poznaje učenike (“The teacher knows the students”). You wouldn’t say poznaje čija je bilježnica.

How would I say it in the past or future?

Past: Učitelj je znao čija je bilježnica. (Notice both clitics je: one for the auxiliary of “know” and one for “is” in the embedded clause.)
Future: Učitelj će znati čija je bilježnica. (će is also a clitic and goes to second position in its clause.)

Are there articles like “the” or “a” in Croatian?

No. Croatian has no articles. Učitelj can mean “a teacher” or “the teacher,” and bilježnica can mean “a notebook” or “the notebook,” depending on context.

Pronunciation tips for učitelj, čija, bilježnica?

č = “ch” in “church” (voiceless): čija ≈ “CHEE-ya”
ž = “s” in “measure”: bilježnica has ž
- n: “bi-LYEH-zh-nee-tsa”
lj = palatal “ly” (like Italian “gli”): učiTELJ ends with -lj (a single sound), ≈ “OO-chee-tely”

Does učitelj imply a male teacher? What about a female teacher?

Učitelj is masculine. For a female teacher, use učiteljica: Učiteljica zna čija je bilježnica. In many contexts učitelj can be used generically, but if gender matters, choose the matching form.