Я только что открыл окно, потому что в комнате душно.

In this sentence, только что is a fixed expression meaning just now / a moment ago.

So:

• Я только что открыл окно = I just opened the window

Even though the individual words are:

• только = only
• что = what / that

together they often function as one expression meaning very recently.

Examples:

• Я только что пришёл. = I just arrived.
• Она только что позвонила. = She just called.

So it is best to learn только что as a set phrase.

Открыл is the past tense of the perfective verb открыть.

Here that makes sense because the sentence describes a completed action:

• I have opened / I just opened the window

Why not the others?

• открываю = I am opening / I open
  This is present tense, so it does not match just opened.
• открывал = past tense of the imperfective verb открывать
  This would focus more on the process, repetition, or general fact of opening, not the single completed result.

Compare:

• Я открыл окно. = I opened the window. / the action is completed
• Я открывал окно. = I was opening / I used to open / I did open (depending on context, but without emphasizing the completed result in the same way)

Because the speaker is talking about one completed action that has just happened, открыл is the natural choice.

In Russian, past tense verbs agree with the speaker’s gender in the singular.

So:

• я открыл = I opened said by a male
• я открыла = I opened said by a female

The sentence as written assumes the speaker is male.
If the speaker were female, the sentence would be:

• Я только что открыла окно, потому что в комнате душно.

This is one of the things English speakers often notice, because English past tense verbs do not change for gender.

It is in the accusative — it just happens to look the same as the nominative.

The verb открыть takes a direct object, so window must be in the accusative case.

The noun окно is:

• nominative singular: окно
• accusative singular: окно

For many inanimate neuter nouns, nominative and accusative are identical.

So:

• Окно открыто. = The window is open.
  (окно = nominative, subject)
• Я открыл окно. = I opened the window.
  (окно = accusative, object)

The form is the same, but the function is different.

Because в комнате means in the room as a location, not motion into the room.

Russian uses different cases after в depending on meaning:

• в + accusative = motion into
• в + prepositional = location in

So:

• в комнату = into the room
• в комнате = in the room

In your sentence, the meaning is it is stuffy in the room, so this is about location. That is why Russian uses the prepositional case:

• комната → в комнате

Потому что means because.

So the structure is:

• Я только что открыл окно = I just opened the window
• потому что в комнате душно = because it’s stuffy in the room

In Russian, a comma is normally placed before потому что when it introduces a subordinate clause.

That is why you get:

• Я только что открыл окно, потому что в комнате душно.

This is very standard Russian punctuation.

Душно is not a normal adjective here. It is a predicative word (sometimes called a category of state word), used to describe a condition or environment.

It means:

• it is stuffy
• the air feels close / oppressive

In Russian, words like this are very common:

• холодно = it is cold
• жарко = it is hot
• темно = it is dark
• трудно = it is difficult
• душно = it is stuffy

So:

• В комнате душно. = It is stuffy in the room.

English usually needs it is, but Russian can express this state without a subject like it.

Because in Russian, the verb to be is normally omitted in the present tense.

English says:

• It is stuffy in the room.

Russian simply says:

• В комнате душно.

There is no present-tense form of быть used here.

This happens very often:

• Он дома. = He is at home.
• Она врач. = She is a doctor.
• Сегодня холодно. = It is cold today.

So the sentence does not need a separate word for is.

Yes. Russian word order is more flexible than English, although the basic version here is very natural.

Original:

• Я только что открыл окно, потому что в комнате душно.

You could also say:

• Потому что в комнате душно, я только что открыл окно.
• Окно я только что открыл, потому что в комнате душно.

These alternatives are possible, but they change the focus or emphasis.

The original version is the most neutral:

1. state the action
2. then give the reason

That is why it is a good default sentence for learners.

Yes, often it could.

Russian frequently drops personal pronouns when the verb form already makes the subject clear.

So in context, a speaker might say:

• Только что открыл окно, потому что в комнате душно.

That still means:

• I just opened the window, because it’s stuffy in the room.

However, Я is perfectly normal and can be included for:

• clarity
• emphasis
• contrast

For example:

• Я только что открыл окно, а не ты. = I just opened the window, not you.

So the pronoun is optional in many contexts, but not wrong at all.

Yes, but its position can slightly affect emphasis.

The most neutral position here is:

• Я только что открыл окно.

You may also hear:

• Я открыл окно только что.

Both mean I just opened the window, but the first version is usually more natural in neutral speech.

Russian allows some movement of adverbs, but learners should usually start with:

• subject + только что + past verb + object

So this sentence is a very good model:

• Я только что открыл окно, потому что в комнате душно.