Перед сном я сняла покрывало и открыла окно, потому что в комнате было душно.

Перед сном literally means before sleep. In natural English, that usually becomes before going to bed or before sleeping.

The preposition перед usually takes the instrumental case, so сон becomes сном.

• сон = sleep, a dream
• перед сном = before sleep / before bedtime

This is a very common Russian expression.

Because after перед, Russian normally uses the instrumental case.

So:

• сон = nominative
• сном = instrumental

This is one of the core things Russian learners need to get used to: prepositions often require specific cases.

In the past tense, Russian verbs agree with the subject in gender and number.

Here the subject is я, but Russian past-tense verbs still show whether the speaker is male or female:

• я снял, я открыл = a male speaker
• я сняла, я открыла = a female speaker

So this sentence is being said by a woman.

Because the sentence describes completed actions that happened before sleep:

• she removed the blanket
• she opened the window

Russian past tense is very commonly used this way for completed events in a sequence.

This is about aspect.

• снять, открыть = perfective
• снимать, открывать = imperfective

Here the speaker is talking about single, completed actions, so perfective is the normal choice:

• сняла = removed
• открыла = opened

If you used снимала / открывала, it would suggest an ongoing, repeated, or background action, which does not fit as well here.

Покрывало usually means a bedspread, cover, or sometimes a blanket-like cover on a bed.

Depending on context, English might translate it differently:

• bedspread
• cover
• sometimes blanket

Since the meaning is already given to the learner, the important thing is that it is something being removed from the bed.

Because покрывало is the direct object of сняла.

For an inanimate neuter noun, the accusative singular looks the same as the nominative singular:

• nominative: покрывало
• accusative: покрывало

So even though it is the object, its form does not change here.

For the same reason as покрывало.

Окно is an inanimate neuter noun, and in the singular, its accusative form is the same as its nominative form:

• nominative: окно
• accusative: окно

Since открыла takes a direct object, окно is in the accusative, but it looks unchanged.

Because в means in, and when it refers to location, it normally takes the prepositional case.

• комната = room
• в комнате = in the room

So:

• в комнату = into the room (motion, accusative)
• в комнате = in the room (location, prepositional)

Душно means something like:

• stuffy
• airless
• close
• oppressively warm because there is not enough fresh air

It does not simply mean hot. A room can be тёплая or жаркая, but душно specifically suggests uncomfortable air and lack of ventilation.

That is why opening the window makes sense.

Because Russian often uses an impersonal construction for environmental conditions.

• было душно = it was stuffy

There is no real subject like English it. Russian just uses:

• past tense of быть in neuter singular: было
• predicative adverb/state word: душно

This is very common:

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

Because было душно is not agreeing with комната as a normal adjective would.

This is an impersonal statement about the situation:

• в комнате было душно = in the room it was stuffy

The word душно is not a regular adjective modifying комната. It is a word of state, and the whole expression uses the neuter singular past form было.

If you wanted to describe the room as a noun with an adjective, you would build the sentence differently.

Because Russian often leaves out repeated subjects when they are obvious.

So:

• я сняла покрывало и открыла окно
  means
• I removed the blanket and opened the window

The subject я applies to both verbs. Repeating it would usually sound unnecessary unless you wanted extra emphasis.

Because потому что introduces a subordinate clause meaning because.

So the sentence is divided into:

• main clause: Перед сном я сняла покрывало и открыла окно
• subordinate clause: потому что в комнате было душно

Russian normally uses a comma before потому что.

Yes. Russian word order is fairly flexible, although the neutral version here is very natural.

For example, you could also say:

• Я перед сном сняла покрывало и открыла окно, потому что в комнате было душно.
• Потому что в комнате было душно, перед сном я сняла покрывало и открыла окно.

The basic meaning stays the same, but the emphasis shifts slightly.

The original sentence is a very normal, straightforward way to say it.