Learn Hardcore Russian: Открой окно, если в комнате душно. - Open the window if it is stuffy in the room.

Открой is the imperative (command/request form) of открыть (to open, perfective aspect). It addresses one person informally (ты implied). It means Open (it)! / Open (the)…! as a single, completed action.

Why is the verb открыть perfective here—could it be открывай instead?

Yes, both are possible, but the nuance changes:

Открой окно (perfective) = open the window (once), achieve the result (window becomes open).
Открывай окно (imperfective imperative) can sound like start opening / be opening or do it regularly/whenever (more process/habit/ongoing feel), and can also be used as a softer “go ahead and open it” depending on context. In this sentence, the idea is usually a one-time action triggered by a condition, so Открой fits well.

What’s the difference between окно and окна or окну here—what case is окно?

Окно here is accusative singular, which for neuter nouns like окно looks the same as nominative: окно. It’s the direct object of Открой (open what? → the window).

Why is there a comma before если?

In Russian, a subordinate clause introduced by если (if) is normally separated by a comma:

Открой окно, если в комнате душно. So the comma marks the boundary between the main clause (Open the window) and the conditional clause (if it’s stuffy in the room).

Can the word order change? For example: Если в комнате душно, открой окно.

Yes, that’s completely natural. Both are correct:

Открой окно, если в комнате душно.
Если в комнате душно, открой окно. The second version foregrounds the condition (If it’s stuffy…) a bit more, similar to English when you start with If….

What does душно mean grammatically—why isn’t it an adjective like душная?

Душно is a category of words often called predicatives or words of state (similar to холодно, жарко, темно). They describe a general state rather than a thing. The structure is impersonal: there is no grammatical subject like it in English. So в комнате душно literally means something like in the room (it is) stuffy, but Russian doesn’t need it is.

Why is it в комнате and not в комнатe (or another case)? What case is комнате?

В комнате uses в + prepositional case to mean in (a location).
Комната → prepositional singular комнате.

в комнате = in the room (location) If you meant motion into the room, you’d often use в + accusative:
в комнату = into the room

Is Открой окно polite? How would I say it more politely or formally?

Открой окно is informal (to one person you address as ты). Politer/formal options:

Откройте окно, если в комнате душно. (imperative plural/formal вы) Softer/request-like versions:
Открой(те), пожалуйста, окно…
Не мог(ли) бы вы открыть окно, если в комнате душно? (more polite, conditional request)

Does если always mean “if,” and can it also mean “whether”?

Если most commonly means if (a condition), like here. It can sometimes function close to whether in some contexts, but Russian often uses ли for “whether”:

Я не знаю, душно ли в комнате. = I don’t know whether it’s stuffy in the room. In this sentence, если is clearly the conditional if.

How is this sentence pronounced, and where is the stress?

Common stress:

Откро́й окно́, е́сли в ко́мнате ду́шно. Notes:
ё is always stressed, but here it’s Открой with о́ stress on the second syllable: -кро́й.
окно́ stress is on the last syllable.
ду́шно stress is on ду́-.

Could I drop в комнате and just say если душно?

Yes. Если душно, открой окно. is natural and common when the location is obvious (e.g., you’re already in the room). Adding в комнате just makes it explicit: if it’s stuffy in the room.

Is there any difference between окно and форточка here?

Yes:

окно = window (the whole window)
форточка = a small vent window / little pane for airing (common in older window designs) So Открой окно suggests opening the main window; Открой форточку suggests a smaller ventilation opening.