В вагоне было тесно и душно, поэтому я расстегнула куртку, как только села.

Because it means in the carriage / in the train car.

After в meaning in, Russian normally uses the prepositional case for location:

• вагон = carriage, train car
• в вагоне = in the carriage

Compare:

• в вагон = into the carriage (motion, accusative)
• в вагоне = in the carriage (location, prepositional)

So here the speaker is describing the conditions inside the carriage, not movement into it.

Вагон usually means a railway carriage / train car. In this sentence, в вагоне most naturally means in the train car or in the carriage.

Depending on context, вагон can sometimes refer to other kinds of cars/wagons, but for a learner, train carriage is the safest interpretation here.

Because this is an impersonal sentence.

Russian often uses было in the neuter singular when describing a general situation or condition, especially with words like:

• холодно = cold
• темно = dark
• тесно = cramped
• душно = stuffy

So:

• В вагоне было тесно и душно = It was cramped and stuffy in the carriage

There is no normal grammatical subject like вагон был... here. The sentence is about the state of the environment, not about a noun agreeing with the verb.

Here they are words of state (often called predicatives), not ordinary adjectives modifying a noun.

They describe a situation:

• тесно = cramped, crowded, with little space
• душно = stuffy, close, lacking fresh air

That is why the Russian structure is not literally the carriage was cramped and stuffy in the same way English uses adjectives. Russian often says something more like:

• In the carriage, it was cramped and stuffy.

These words are very common in impersonal sentences:

• Здесь тесно. = It’s cramped here.
• В комнате душно. = It’s stuffy in the room.

They describe two different discomforts:

• тесно = there is not enough space; it feels cramped or crowded
• душно = the air is bad / heavy / stuffy; it feels hard to breathe comfortably

So the sentence means the carriage was uncomfortable both because it was crowded/cramped and because it was stuffy.

Because Russian often does not use a dummy subject like English it.

English says:

• It was stuffy.
• It was cramped.

Russian simply says:

• Было душно.
• Было тесно.

This is a very common difference between English and Russian. In many weather, time, and condition expressions, Russian uses an impersonal structure with no explicit subject.

Расстегнула is the past tense feminine singular form of расстегнуть = to unfasten / unzip / undo.

The ending -ла shows that the speaker is female:

• я расстегнул = I unfastened/unzipped (male speaker)
• я расстегнула = I unfastened/unzipped (female speaker)

So from this sentence alone, we know the speaker is a woman.

Because it is the direct object of расстегнула.

The speaker did something to the jacket:

• куртка = jacket (nominative)
• расстегнула куртку = unfastened/unzipped the jacket (accusative)

For feminine nouns ending in -а, the accusative singular usually changes to -у:

• куртка → куртку

It can mean either, depending on the jacket.

Расстегнуть is a general verb meaning to undo/unfasten something that is fastened:

• unzip a jacket
• unbutton a coat
• undo a clasp, buckle, etc.

So расстегнула куртку is best understood as opened/unfastened her jacket. If the meaning shown to the learner says unzipped, that is a natural translation if the jacket has a zipper.

Поэтому means therefore / so / that’s why.

It connects the two ideas:

• В вагоне было тесно и душно = It was cramped and stuffy in the carriage
• поэтому я расстегнула куртку = so I unfastened my jacket

It introduces the result of the previous situation.

The comma before поэтому is normal because it links two clauses.

Как только means as soon as.

So:

• как только села = as soon as (I) sat down

It introduces an action that happened immediately after another one.

Examples:

• Как только я пришёл, я позвонил. = As soon as I arrived, I called.
• Как только она села, она открыла книгу. = As soon as she sat down, she opened the book.

Russian often omits the subject when it is easily understood from context.

Here the subject is clearly the same я from the main clause:

• поэтому я расстегнула куртку, как только села

The form села is also feminine singular past, so it matches the same female speaker.

A fuller version would be:

• ..., как только я села.

But leaving out я sounds perfectly natural in Russian because the subject is already obvious.

Села is the feminine past tense of сесть = to sit down.

Again, the ending shows the speaker is female:

• я сел = I sat down (male speaker)
• я села = I sat down (female speaker)

It also shows a completed action: to sit down, not to be sitting.

That fits well with как только:

• как только села = as soon as I sat down

Because both actions are presented as single completed events.

• расстегнуть → расстегнула = unfastened / opened up
• сесть → села = sat down

This makes sense in the context:

• she sat down
• she immediately unfastened her jacket

If imperfective forms were used, the meaning would change and sound less natural here, because the sentence is about specific completed actions, not ongoing processes.

Yes. Russian word order is flexible, although the original sentence is very natural.

For example, you could also say:

• В вагоне было тесно и душно, поэтому, как только я села, я расстегнула куртку.
• Как только села, я расстегнула куртку, потому что в вагоне было тесно и душно.

The original version is smooth and common because it presents:

1. the situation
2. the result
3. the exact moment when the result happened

So the original word order feels very natural in narration.

Yes, but that would change the focus slightly.

• В вагоне было тесно и душно = the carriage itself felt cramped and stuffy; this describes the general environment
• Мне было душно = I felt stuffy / I felt hot and short of air

The actual sentence focuses on the conditions in the carriage as a whole, not only the speaker’s personal sensation. Of course, those conditions are also the reason she unfastened her jacket.