В салоне машины было душно, поэтому я открыл окно.

Because душно is not an adjective here. It is a word of state, used in impersonal sentences to describe an environment or condition: it was stuffy, it was cold, it was dark, etc.

Russian often uses this pattern:

• было душно = it was stuffy
• было холодно = it was cold
• было темно = it was dark

Since there is no real grammatical subject, the verb in the past tense takes the default neuter singular form было.

So В салоне машины было душно literally works like:

• In the car interior, it was stuffy

Because this is an impersonal sentence, which is very common in Russian.

English often uses a dummy subject like it:

• It was stuffy in the car

Russian usually does not need an equivalent dummy subject. It simply says:

• В салоне машины было душно

So the sentence is complete even without a word meaning it.

There are two different case forms here:

• в салоне: prepositional case, because в with location means in/inside
• машины: genitive case, because it means of the car

So:

• салон = interior / cabin
• в салоне = in the interior
• машины = of the car

Together:

• в салоне машины = in the car’s interior / inside the car

Both are possible.

• в машине = in the car
• в салоне машины = in the interior/cabin of the car

Салон is a bit more specific. It refers to the inside passenger area, not just the car in general. In many everyday situations, a native speaker could simply say:

• В машине было душно, поэтому я открыл окно.

That would sound completely natural too.

Here салон means the interior or cabin of a vehicle.

This word can also be used in other contexts, so learners may already know it from expressions like:

• салон красоты = beauty salon
• мебельный салон = furniture showroom

But in car-related language, салон машины means the passenger compartment or interior.

Russian often expresses X of Y with a noun in the genitive.

So:

• салон машины = the interior of the car
• окно дома = the window of the house
• дверь комнаты = the door of the room

You could build an adjective in some cases, but that is not the normal way here. Салон машины sounds natural and straightforward.

You may also hear:

• салон автомобиля

This is slightly more formal than салон машины.

Поэтому means therefore, so, or that’s why.

It connects the first clause and the result:

• В салоне машины было душно = It was stuffy inside the car
• поэтому я открыл окно = so I opened the window

It signals a result/consequence, not a cause.

Compare:

• потому что = because
• поэтому = therefore / so

For example:

• Я открыл окно, потому что в салоне машины было душно.
  = I opened the window because it was stuffy inside the car.

• В салоне машины было душно, поэтому я открыл окно.
  = It was stuffy inside the car, so I opened the window.

Because there are two clauses here:

• В салоне машины было душно
• поэтому я открыл окно

In Russian, clauses like this are normally separated by a comma. Since поэтому introduces the result of the first clause, the comma is expected.

Because открыл is the perfective past tense, and it presents the action as a completed whole:

• открыл окно = opened the window

That fits this sentence well: it was stuffy, so the speaker did one completed action in response.

Compare:

• открыл = opened / did open
• открывал = was opening / used to open / opened repeatedly / opened at some point but with focus on process or repetition

In this context, открыл is the natural choice because the sentence is about a specific result.

Because окно is a neuter inanimate noun, and for neuter inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: окно
• accusative: окно

Since открыть takes a direct object, окно is in the accusative here, but its form does not change.

Yes, absolutely. That version is also natural, but the nuance is a little different.

• В салоне машины было душно focuses on the condition inside the car itself.
• Мне было душно в салоне машины focuses more on my feeling: I felt stuffy / I was uncomfortable because it was stuffy.

Both can lead naturally to поэтому я открыл окно.

Yes. Russian word order is flexible, although the basic meaning stays the same and the emphasis changes.

For example:

• В салоне машины было душно, поэтому я открыл окно.
  Neutral, very natural.

• Я открыл окно, потому что в салоне машины было душно.
  Also very natural, but now the action comes first.

• Окно я открыл, потому что в салоне машины было душно.
  This gives extra emphasis to the window.

So the original order is a normal, neutral way to say it, but Russian allows other arrangements depending on what you want to emphasize.