В комнате стало теплее, когда я закрыл окно.

The room became warmer when I closed the window.

Breakdown of В комнате стало теплее, когда я закрыл окно.

• я: I
• в: in
• комната: the room
• окно: the window

Questions & Answers about В комнате стало теплее, когда я закрыл окно.

Why is it в комнате, not в комната?

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.

Why does the sentence use стало?

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.