В комнате стало теплее, когда я закрыл окно.

Because after в when you mean in a place, Russian normally uses the prepositional case.

• комната = room
• в комнате = in the room

So here:

• В комнате = In the room

If в showed movement into a place, you would usually use the accusative instead:

• в комнату = into the room

So the sentence starts with location, not movement, which is why комнате is correct.

Стало is the past tense of стать used in an impersonal construction.

The pattern is:

• стало + comparative = it became + more...

So:

• стало теплее = it became warmer

Russian often uses this kind of neuter singular past form in sentences where English would use it as a dummy subject:

• Стемнело. = It got dark.
• Похолодало. = It got colder.
• Стало теплее. = It became warmer.

There is no real subject here like the room became warmer. Russian just uses the impersonal form стало.

Because теплее is the comparative form and the sentence is talking about a change: the room became warmer than before.

Compare:

• тёплый = warm, used like an adjective with a noun
  
  • тёплая комната = a warm room
• тепло = warm, used predicatively/adverbially
  
  • В комнате тепло. = It is warm in the room.
• теплее = warmer
  
  • В комнате стало теплее. = It became warmer in the room.

So теплее is used because the idea is not just warm, but warmer than before.

Because this is a very common Russian way to describe changes in weather, temperature, light, mood, and similar states.

English often uses it:

• It got warmer
• It became dark

Russian often uses an impersonal sentence instead:

• Стало теплее
• Стемнело

There is no explicit subject, but the meaning is perfectly natural. In this sentence, В комнате tells you where this change happened.

Закрыл is the perfective past form, and it shows a completed action: I closed the window.

That fits the meaning here because the warming happened after the action was completed:

• когда я закрыл окно = when I closed the window

If you used закрывал, that would usually suggest an ongoing, repeated, or process-like action, which does not fit as well here.

So the perfective verb is natural because:

1. the action happened once,
2. it was completed,
3. it triggered the result: it became warmer.

Because окно is the direct object of закрыл, so it is in the accusative case.

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

So:

• nominative: окно
• accusative: окно

That is why the form does not visibly change.

Compare with a feminine noun:

• я закрыл дверь = I closed the door

Here дверь also happens to look the same in nominative and accusative, but for other nouns the change may be more obvious.

Here когда means when.

It introduces a time clause:

• когда я закрыл окно = when I closed the window

In this sentence, it can also feel a bit like once or after, because the action of closing the window leads to the room getting warmer.

So the whole sentence means something like:

• It became warmer in the room when I closed the window
• or more naturally in English: The room got warmer when I closed the window

Yes. That is a very natural English translation.

Even though the Russian literally uses an impersonal structure:

• В комнате стало теплее = It became warmer in the room

English often prefers:

• The room got warmer
• The room became warmer

So the Russian structure and the most natural English structure are not identical grammatically, but they express the same idea.

Yes, the word order can change. Russian word order is flexible.

This version:

• В комнате стало теплее, когда я закрыл окно.

puts the location first, which sounds natural and helps set the scene: in the room.

Other orders are possible, for example:

• Когда я закрыл окно, в комнате стало теплее.
• Стало теплее в комнате, когда я закрыл окно.

These all mean roughly the same thing, but the emphasis changes a little.

• В комнате first = focus on the place
• Когда я закрыл окно first = focus on the timing/action
• Стало теплее first = focus on the change itself

Yes, but it would mean something different.

• В комнате стало теплее = It became warmer in the room
  This emphasizes a change.
• В комнате было теплее = It was warmer in the room
  This describes a state, not the change into that state.

So if you want to say the room got warmer after you closed the window, стало теплее is the right choice.

Not necessarily. In Russian, a comparative form like теплее can compare something to an earlier state, even if the second thing is not explicitly mentioned.

So here:

• стало теплее = it became warmer

The implied comparison is:

• warmer than before

You do not need to say than before explicitly. Russian often leaves that understood from context.

If you wanted to compare it to another place, you could make that explicit:

• В комнате стало теплее, чем на улице. = It became warmer in the room than outside.

Yes, it is completely natural.

It uses several very common Russian patterns:

• в + prepositional for location: в комнате
• impersonal стало + comparative for change of state: стало теплее
• когда + past tense for a time clause: когда я закрыл окно
• perfective past for a completed action: закрыл

So this is a very useful model sentence for everyday Russian.