Если в комнате душно, то я открываю форточку и выключаю обогреватель.

Because в can take either the prepositional or the accusative, depending on meaning:

• в комнате = in the room (location, no movement) → prepositional
• в комнату = into the room (movement toward) → accusative

So in this sentence, the meaning is location: If it’s stuffy in the room..., so в комнате is correct.

Душно means something like stuffy, airless, or close in the sense of unpleasant warm air.

It is not a normal adjective here. It is a predicative adverb / category of state word, which is often used in impersonal sentences:

• Мне холодно. = I am cold.
• Здесь темно. = It is dark here.
• В комнате душно. = It is stuffy in the room.

Because of that, it does not change for gender, number, or case.

Russian often uses impersonal constructions where English uses it is.

English:

• If it is stuffy in the room...

Russian:

• Если в комнате душно...

Russian does not need a dummy subject like English it. The meaning is complete without it.

Если means if and introduces a condition.

So:

• Если в комнате душно... = If the room is stuffy...

It works much like English if.

То corresponds to the then idea in if ... then ....

So:

• Если в комнате душно, то я открываю форточку...
• Literally: If it is stuffy in the room, then I open the vent window...

Important points:

• то is often optional
• It is used to make the sentence structure clearer or more emphatic
• Very common after an если clause, especially in longer sentences

So both are possible:

• Если в комнате душно, я открываю форточку...
• Если в комнате душно, то я открываю форточку...

Both are correct.

In Russian, the present tense of an imperfective verb is often used for habitual or repeated actions.

So this sentence means:

• If it’s stuffy in the room, I open the little window and turn off the heater.

This is not necessarily happening right now. It can mean:

• Whenever / if this situation happens, that is what I do.

This is very natural in Russian.

They are imperfective because the sentence describes a usual response or a repeated action in a general situation.

• открывать / открываю = to open, be opening, open habitually
• выключать / выключаю = to turn off habitually

If you wanted to describe a single completed future action, Russian would usually use the perfective future:

• Если в комнате будет душно, я открою форточку и выключу обогреватель. = If the room is stuffy / becomes stuffy, I’ll open the vent window and turn off the heater.

So:

• открываю, выключаю = habitual / repeated
• открою, выключу = one future occasion

Форточка is a small opening window or vent window, especially the kind often built into a larger window in older Russian-style apartments.

It appears here as форточку because it is the direct object of открываю.

Dictionary form:

• форточка

In the sentence:

• открываю форточку = I open the vent window

This is a very culturally specific and common Russian household word.

Because форточку is the accusative singular form.

The verb открывать / открыть takes a direct object, and direct objects are often in the accusative:

• открывать что? → форточку

Since форточка is a feminine noun ending in -а, its accusative singular changes to -у:

• nominative: форточка
• accusative: форточку

Because обогреватель is a masculine noun, and for most inanimate masculine nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: обогреватель
• accusative: обогреватель

And since выключаю takes a direct object:

• выключаю что? → обогреватель

That is why the form does not change.

Обогреватель means heater or space heater.

It comes from the idea of heating/warming:

• греть = to warm, heat
• обогревать = to heat, warm
• обогреватель = a device that heats

So выключаю обогреватель means I turn off the heater.

Yes, it can often be omitted.

Russian frequently leaves out subject pronouns when the verb form already makes the subject clear.

• я открываю = I open
• открываю by itself already strongly suggests I

So both are possible:

• Если в комнате душно, то я открываю форточку и выключаю обогреватель.
• Если в комнате душно, то открываю форточку и выключаю обогреватель.

Including я can add clarity, contrast, or emphasis.

Russian word order is more flexible than English word order. The sentence given is neutral and natural:

• Если в комнате душно, то я открываю форточку и выключаю обогреватель.

But other orders are possible, depending on emphasis:

• Если в комнате душно, я открываю форточку и выключаю обогреватель.
• Я, если в комнате душно, открываю форточку и выключаю обогреватель.

The original order is probably the best neutral version for a learner.

Because Если в комнате душно is a subordinate if-clause, and in Russian such clauses are normally separated by a comma from the main clause.

So:

• Если в комнате душно, то я открываю...

This comma is required.

Yes, very often.

Because the main verbs are in the present imperfective, the sentence naturally suggests a habitual pattern:

• If / whenever it gets stuffy in the room, I open the vent window and turn off the heater.

So although если literally means if, the whole sentence can sound like a repeated habit, close to whenever in English.

Yes.

• Если в комнате душно... = If it is stuffy in the room...
• Если в комнате становится душно... = If it starts getting stuffy in the room...

The second version emphasizes the change of state. The original just describes the condition itself.

Yes, but the meaning changes slightly.

• окно = window
• форточка = small vent window / small opening pane

So:

• открываю окно = I open the window
• открываю форточку = I open the little vent window

The original sentence is more specific and sounds very natural in a Russian home context.