Questions & Answers about Мы делим одну комнату.
Because комната is a feminine noun, and here it is the direct object of the verb, so it must be in the accusative case.
- Nominative: одна комната = one room
- Accusative: одну комнату = one room (as the object)
So одна changes to одну to match the case of комнату.
For the same reason: it is the direct object of делим.
The verb делить takes a direct object, so комната must go into the accusative singular:
- Nominative: комната
- Accusative: комнату
This is a very common pattern with feminine nouns ending in -а:
- книга → книгу
- машина → машину
- комната → комнату