Learn Hardcore Croatian: Nemoj otvoriti prozor kad je vani oluja. - Don’t open the window when there is a storm outside.

Nemoj + infinitive (very common, neutral, often a bit more “advising”): Nemoj otvoriti prozor.
Ne + imperative form (also correct; can sound a bit more direct/commanding): Ne otvori prozor. (perfective) / Ne otvaraj prozor. (imperfective)

Nemoj is the imperative of ne htjeti (don’t want → used idiomatically as don’t do). With it, Croatian typically uses the infinitive: nemoj otvoriti.

Why is the verb otvoriti (perfective) and not otvarati (imperfective)?

The aspect changes the nuance:

otvoriti (perfective) = open it (once), perform the action to completion
→ Nemoj otvoriti prozor = Don’t open the window (at all / don’t do that action).
otvarati (imperfective) = be opening / open repeatedly / be in the process
→ Nemoj otvarati prozor = Don’t open the window (habitually / repeatedly) or don’t be opening it.

With storms, the “one-time action” warning fits well, so otvoriti is very natural.

What case is prozor in, and why?

Prozor is in the accusative (direct object) because otvoriti takes a direct object: open what? → prozor.

For many masculine inanimate nouns, nominative = accusative in the singular, so prozor looks the same in both cases.

Why is it kad je vani oluja and not kad je oluja vani? Is word order flexible?

Word order is fairly flexible, but it affects emphasis and what sounds most natural.

kad je vani oluja sounds like: when outside there is a storm (focus on outside conditions)
kad je oluja vani is also possible, but often sounds slightly more “statement-like,” with oluja more foregrounded.

Croatian often places vani early in this kind of weather context.

What exactly does kad mean here, and how is it different from kada?

kad = when (very common in everyday speech).
kada is basically the same meaning, often a bit more formal or explicit.

So these are both correct:

Nemoj otvoriti prozor kad je vani oluja.
Nemoj otvoriti prozor kada je vani oluja.

Why is there je in kad je vani oluja?

je is the present tense of biti (to be): (it) is.

Croatian commonly uses biti in “there is/it is” weather-style sentences:

Vani je oluja. = Outside, there is a storm / It’s storming outside.

In English you might omit is in some constructions, but Croatian needs the verb.

Is oluja the subject here? Why is it in nominative?

Yes—grammatically, oluja (storm) is in the nominative and acts like the subject of je:

Oluja je vani. (Storm is outside.)
Vani je oluja. (Outside is a storm.)

Croatian expresses “there is X” in the present using biti + nominative in many cases like this.

Can I replace vani with something else, like napolju?

Yes. Common options:

vani = outside (very common; widely understood)
van can also occur in some contexts (often “out” rather than “outside”)
napolju = outside (common especially in Serbian; still widely understood in Croatian, but vani is more standard/typical in Croatian usage)

So a natural Croatian alternative is:

Nemoj otvoriti prozor kad je vani oluja. (most typical)

What’s the difference between prozor and prozore? Could it be plural?

prozor = window (singular accusative)
prozore = windows (plural accusative)

Both are possible depending on the situation:

Nemoj otvoriti prozor... = Don’t open the (one) window...
Nemoj otvoriti prozore... = Don’t open the windows...

How would I say this to more than one person?

Change nemoj (singular) to nemojte (plural/polite):

Nemojte otvoriti prozor kad je vani oluja. = Don’t open the window when there’s a storm outside. (to you all / formal you)

Croatian uses nemojte both for plural you and polite singular you.

Could I also say Dok je vani oluja instead of Kad je vani oluja?

Yes, but the meaning shifts slightly:

kad = when (at the time that; can be general or conditional)
dok = while (emphasizes “during the time that”)

So:

Nemoj otvoriti prozor kad je vani oluja. = Don’t open the window when/whenever there’s a storm outside.
Nemoj otvoriti prozor dok je vani oluja. = Don’t open the window while the storm is going on outside.

Both can work; dok feels more explicitly “during the storm.”

How is this sentence pronounced, especially nemoj and oluja?

Approximate pronunciation (very rough, English-friendly):

Nemoj ≈ NE-moy (two syllables)
otvoriti ≈ ot-VO-ree-tee
prozor ≈ PRO-zor
kad je ≈ kad yeh (the j is like English y)
vani ≈ VA-nee
oluja ≈ o-LOO-ya (the j again like y)

Also note: Croatian r is “rolled/tapped,” especially in careful speech.