Learn Finnish: Ostimme auton osamaksulla. - We bought the car in instalments.

While this lesson focuses on using the adessive for exact prices, it is also used to express the method of payment in a transaction. Just as you would say viidellä tonnilla (for five grand), you say osamaksulla (by instalment).

How does the word osamaksu break down?

It is a compound of osa (part) and maksu (payment or fee). Literally, it translates to "part-payment".

Why is the object auton ending in -n instead of the partitive autoa?

Because the action is complete and covers the whole object. You bought the entire car, not just an unspecified amount of it, and the buying process is finished. Therefore, it takes the total object case, which looks identical to the genitive.

Can I use this same adessive structure to say I paid with cash or a card?

Yes! You use the exact same adessive structure for other payment methods: käteisellä (with cash) and kortilla (with a card).