Learn Finnish: Ostin uuden kaksioni ystävälliseltä välittäjältä. - I bought my new one-bedroom apartment from a friendly real estate agent.

The verb ostaa (to buy) requires a "from" case when stating the seller or source. Because you are buying from a person (an agent), you use the ablative case (-lta/-ltä). If you bought it from a physical place like a store, you would use the elative case (-sta/-stä), such as kaupasta. Remember that adjectives must match the noun's case, so ystävällinen changes to ystävälliseltä.

Why does the adjective uuden end in -n, but kaksioni does not?

The verb ostin requires its total object to be in the genitive case, which normally adds an -n ending. However, when you attach a possessive suffix like -ni (my) to a noun, it entirely replaces the genitive -n. So, kaksion + -ni becomes kaksioni. The adjective uusi doesn't get a possessive suffix, so it keeps the standard genitive -n ending and becomes uuden.

If kaksi means "two", why does kaksio mean a "one-bedroom apartment"?

In Finland, apartments are categorized by the total number of living spaces, not counting the kitchen and bathroom. A kaksio literally means a "two-roomer", which translates to a living room plus one bedroom. For context, a studio is a yksiö (one room), and a two-bedroom apartment is a kolmio (three rooms).