Learn Hardcore Croatian: Otvorimo prozor jer zrak stvarno treba biti svjež. - Let’s open the window because the air really needs to be fresh.

It’s the first-person plural imperative (hortative) of otvoriti: Otvorimo = “Let’s open.” It’s an inclusive “we” command. You’ll also hear the very common colloquial alternatives Hajdemo/ajmo otvoriti (“Let’s go open”).

Why use the perfective verb (otvoriti → Otvorimo) instead of the imperfective (otvarati → Otvarajmo)?

Commands for a single, completed action use the perfective: Otvorimo (prozor)! = “Let’s open (the window) now.” The imperfective imperative Otvarajmo would suggest a repeated/ongoing activity (“Let’s be opening [it]/keep opening”), which doesn’t fit here.

Why is it "prozor" and not "prozora"?

Prozor is a masculine inanimate noun. As the direct object (accusative singular) of otvoriti, it keeps the nominative form: otvoriti prozor. Masculine animate nouns change in the accusative (e.g., “vidim psa”), but inanimate ones like prozor do not.

Do I need a comma before "jer"?

In formal writing, it’s common to put a comma before jer because it introduces a subordinate clause of reason: Otvorimo prozor, jer zrak stvarno treba biti svjež. In everyday writing you’ll often see it without the comma, as in your sentence. For school/official texts, include the comma.

What’s the difference between "jer" and "zato što"?

Both mean “because.”

jer is the straightforward “because.”
zato što also means “because,” and pairs neatly with zato (“therefore/that’s why”) if you want that cause–effect link: Zato ćemo otvoriti prozor što zrak treba biti svjež. More naturally: Otvorimo prozor zato što zrak stvarno treba biti svjež. Style-wise, zato što often feels a bit more explicit; jer is shorter and very common.

Where can I place "stvarno"?

Croatian adverbs move fairly freely for nuance:

Zrak stvarno treba biti svjež. (neutral: “really” modifies the whole need)
Zrak treba stvarno biti svjež. (emphasis on the state “really being fresh”)
Stvarno zrak treba biti svjež. (fronted for strong emphasis) All are grammatical; choose the one that matches the emphasis you want.

Why "treba biti"? Could I say "mora biti" or "trebao bi biti"?

Yes, with different strengths:

treba biti = “needs to be / should be” (neutral obligation/necessity)
mora biti = “must be” (strong obligation)
trebao bi biti = “ought to be” (softer, more tentative)

Can I say "treba da bude svjež"?

In standard Croatian, prefer the infinitive: treba biti svjež. The treba da + present construction is characteristic of Serbian/Bosnian. Croatians will understand treba da bude, but it’s not standard Croatian usage.

Why is it "svjež," not "svježe/svježi/svježeg"?

After biti (“to be”), predicate adjectives are in the nominative and agree with the subject:

zrak (masculine singular) → svjež. Other forms show different gender/number/case or functions:
svježe = neuter singular or an adverb (“freshly”)
svježi = masculine plural (or the long/definite form in some contexts)
svježeg = genitive/accusative (animate) masculine singular Here, with biti and subject zrak, use svjež.

How do I pronounce "svjež" and "zrak"?

svjež ≈ “svyezh”: j is like English “y,” ž is like the “s” in “measure.”
zrak = “zrak”: roll/trill the r lightly; all letters are pronounced. Tip: Croatian spelling is very phonetic; every letter is sounded.

Can I replace "prozor" with a pronoun, e.g., "Otvorimo ga"?

Yes. ga = “it/him” for masculine singular objects. Place clitic pronouns in the “second position” of the clause:

Otvorimo ga.
Ajmo ga otvoriti. If the noun has just been mentioned, pronouns are natural to avoid repetition.

Is "treba" impersonal here? What happens with plurals?

Croatian has two patterns with trebati:

Impersonal necessity: Treba (+ infinitive/NP) = “it is necessary to …” (fixed treba)
Personal “to need”: subject agrees with the verb. In your sentence, Zrak treba biti svjež behaves like a personal-use pattern (subject = zrak, verb = treba 3sg). With plurals you can say:
Dokumenti trebaju biti potpisani. (personal: “Documents need to be signed.”)
Or impersonal: Dokumente treba potpisati. (“It is necessary to sign the documents.”) Both are common; choose based on what you want to emphasize.

Does "zrak" always mean "air"? I’ve seen it elsewhere.

In Croatian, zrak primarily means “air,” but it also means “ray/beam” (e.g., sunčev zrak = “sunbeam”). In Serbian/Bosnian, “air” is typically vazduh. In Croatian, the collocation for “fresh air” is svjež zrak.

Do we need "biti"? Why not "zrak stvarno treba svjež"?

Yes, you need biti. With adjectives, Croatian uses the copula: treba biti svjež = “needs to be fresh.” Without biti, trebati expects a noun phrase: e.g., Trebamo svjež zrak = “We need fresh air” (different meaning).

How do I make the negative: “Let’s not open the window …”?

Common options:

Nemojmo otvarati prozor… (very common colloquial negative imperative)
Ne otvarajmo prozor… (also used) Using perfective is possible too: Nemojmo otvoriti prozor, but with negatives the imperfective (otvarati) is more frequent if you mean “let’s avoid doing it.”