Learn Finnish: Saimme suuren asuntolainan, joten ostamme tämän omakotitalon omistajalta. - We got a large mortgage, so we are buying this detached house from the owner.

The verb ostaa (to buy) requires you to state who or where you are buying from using the local "from" cases. When buying from a place, you use the elative (-sta/-stä), but because omistaja (owner) is a person, you must use the ablative ending (-lta/-ltä).

Why do both asuntolainan and omakotitalon end in -n?

They are both total objects. We got the entire mortgage (saimme suuren asuntolainan) and we are buying the entire house (ostamme tämän omakotitalon). When a sentence describes a complete action that affects the whole object, the object takes the genitive-accusative -n ending.

How do the compound words asuntolaina and omakotitalo break down?

Asuntolaina combines asunto (housing/apartment) and laina (loan) to mean "mortgage". Omakotitalo means "detached house" and is famously made of three words: oma (own), koti (home), and talo (house) — literally an "own-home-house".

Why does the first half of the sentence use the past tense, while the second half uses the present tense?