Learn Hardcore Russian: Сегодня в комнате душно, поэтому я открою окно. - Today it is stuffy in the room, so I will open the window.

Russian often uses an impersonal sentence for states like “stuffy,” “cold,” “dark,” etc. The idea is “It is stuffy (here),” and the verb быть is normally omitted in the present tense.
So в комнате душно is a complete sentence by itself.

What part of speech is душно, and how does it work grammatically?

Душно is a predicative adverb (категория состояния). It functions as the predicate in an impersonal sentence describing a general state/condition. Common similar words: жарко, холодно, темно, шумно, скучно.

Why do we say в комнате and not something like комната or в комнату?

В комнате uses в + prepositional case to mean “in (a location).”

комната (nominative) would just be “the room” as a subject, which doesn’t fit here.
в комнату uses accusative and means movement “into the room,” not location.

What case is комнате, and how do I recognize it?

комнате is prepositional singular of комната. It appears after many location uses of в/на:

в комнате, на кухне, в городе, etc.

Why is there a comma before поэтому?

Here поэтому introduces the result/consequence (“therefore/so”), and the comma separates two clauses:
1) Сегодня в комнате душно
2) поэтому я открою окно
This punctuation is very common in Russian with cause → result structure.

Is поэтому a conjunction like “because,” or something else?

Поэтому is closer to a result word (“therefore/so/that’s why”), not a cause word.

Cause is usually introduced by потому что (“because”).
Result is often introduced by поэтому (“so/therefore”).

Could I replace поэтому with потому что here?

Not directly without changing the structure. You’d switch the order and make the second clause the cause, e.g.:

Я открою окно, потому что в комнате душно.
Both are natural; they just package the logic differently (result→cause vs cause→result).

Why is я included? Can it be omitted?

Yes, я is often optional because the verb ending already shows the subject: открою = “I will open.”

поэтому открою окно sounds fine and slightly more casual/less emphatic.
Including я can add a bit of emphasis or clarity (“therefore, I will open…”).

Why is the verb открою and not открываю or открою́сь?

открою is perfective future (one completed action: open the window once). That fits the idea of making a decision now.

открываю is imperfective present, usually “I am opening / I open (habitually),” which doesn’t match as well unless you mean you’re doing it right now at this moment.
откроюсь would mean “I will open myself” (reflexive), which isn’t correct for opening a window.

What’s the difference between открыть and открывать in general?

открыть (perfective): one-time, completed result (“open (and it becomes open)”).
открывать (imperfective): process, repeated action, or general ability/habit (“be opening / open regularly”).
In this sentence, the result (the window ends up open) is what matters, so открою is the natural choice.

Why is окно in this form, and what case is it?

окно is the direct object of открою, so it’s in the accusative case. For inanimate neuter nouns, accusative looks the same as nominative: окно → окно.

Is the word order flexible here? Could I say В комнате сегодня душно?

Yes, Russian word order is flexible and changes emphasis:

Сегодня в комнате душно emphasizes today first.
В комнате сегодня душно emphasizes the location first (“In the room, it’s stuffy today”).
Both are correct; the original is very neutral and common.