Я открыла форточку, потому что на кухне было душно.

Открыла is the past tense feminine singular form of the verb открыть (to open).

• открыть = infinitive, to open
• открыл = he opened
• открыла = she / I opened (if the speaker is female)
• открыло = it opened
• открыли = they opened

In Russian past tense, the verb agrees with the gender/number of the subject.

So:

• Я открыл = I opened (said by a man)
• Я открыла = I opened (said by a woman)

Because it is the direct object of the verb открыла.

The dictionary form is форточка. Here it changes to форточку because it is in the accusative singular:

• nominative: форточка
• accusative: форточку

This is very common with feminine nouns ending in -а / -я.

Compare:

• Я вижу книгу.
• Я открыла форточку.

Both книгу and форточку are accusative singular.

Форточка is a very Russian word for a small window pane or small vent window that opens separately, often in a larger window frame.

It is not usually a full-sized window. In many contexts, English translations might use:

• the little window
• the vent window
• the small window pane

So the sentence does not necessarily mean opening a whole window; it often means opening just the small part to let air in.

This is about aspect.

• открыть = perfective
• открывать = imperfective

In this sentence, открыла means the speaker completed the action: she opened the vent window.

Russian often uses the perfective when talking about a single completed action in the past.

So:

• Я открыла форточку = I opened it (completed action)
• Я открывала форточку = I was opening it / I used to open it / I opened it at some point, with less focus on the result

Here, the completed result matters: it was stuffy, so she opened it.

Because потому что introduces a subordinate clause, and in Russian this is normally separated by a comma.

So the sentence has two parts:

• Я открыла форточку
• потому что на кухне было душно

The comma marks the boundary between the main clause and the reason clause.

This is standard Russian punctuation.

Потому что means because.

It is written as two separate words in Russian:

• потому
• что

You should learn it as a fixed expression meaning because.

Example:

• Я ушёл, потому что было поздно.
• I left because it was late.

So in your sentence:

• Я открыла форточку, потому что на кухне было душно.
• I opened the vent window because it was stuffy in the kitchen.

This is an idiomatic preposition choice. In Russian, when talking about being in the kitchen, the usual expression is:

• на кухне

Even though English uses in, Russian often uses на with certain locations, especially rooms or functional spaces in some common expressions.

So:

• на кухне = in the kitchen
• на работе = at work
• на почте = at the post office

You may also hear в кухне in some contexts, but на кухне is the normal everyday choice for location.

Because after на meaning location, Russian usually uses the prepositional case.

The noun changes like this:

• nominative: кухня
• prepositional: на кухне

So:

• кухня = kitchen
• на кухне = in the kitchen

This is a regular pattern for many feminine nouns ending in -я:

• неделя → на неделе
• линия → на линии
• кухня → на кухне

Because this is an impersonal construction.

Russian often describes conditions like cold, hot, dark, quiet, stuffy without a grammatical subject. English often uses it was..., but Russian usually does not use an actual word for it.

So:

• было душно = it was stuffy

Here, было is the past tense neuter singular form used in impersonal sentences.

This structure is very common:

• Было холодно. = It was cold.
• Было темно. = It was dark.
• На кухне было душно. = It was stuffy in the kitchen.

If you said душная кухня, that would mean a stuffy kitchen as a noun phrase, not the same sentence structure.

Душно is not an adjective here. It is a predicative word used to describe a state or condition, similar to words like:

• холодно = cold
• жарко = hot
• темно = dark
• тихо = quiet
• душно = stuffy

These words are often used in impersonal sentences:

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

So душно describes the environment, not a noun directly.

Because Russian has no articles.

So форточку can mean:

• a vent window
• the vent window

and на кухне can mean:

• in a kitchen
• in the kitchen

The exact meaning depends on context.

In this sentence, English naturally uses the kitchen and probably the vent window, but Russian does not need separate words for that.

Yes. Russian word order is fairly flexible, and changing it often changes emphasis, not the basic meaning.

The original sentence:

• Я открыла форточку, потому что на кухне было душно.

Other possible orders:

• Потому что на кухне было душно, я открыла форточку.
• На кухне было душно, поэтому я открыла форточку.

These all express roughly the same idea, but the focus shifts:

• starting with Я открыла... focuses first on the action
• starting with Потому что... focuses first on the reason
• starting with На кухне было душно... sets the scene first

The stress is on -ы-:

• открЫла

This is important, because Russian stress is unpredictable and must be learned with the word form.

A rough pronunciation guide:

• я = ya
• открЫла = at-KRY-la
• фортОчку = far-TOCH-koo
• потомУ что = pa-ta-MOO shto
• на кУхне = na KOO-kh-nye
• бЫло = BY-la
• дУшно = DOOSH-na

Learning the stressed syllable early helps a lot with speaking and listening.