Learn Finnish: Vakuutus maksaa sata euroa vuodessa. - The insurance costs a hundred euros a year.

While English uses "per" or "a" to express rates (like "a hundred euros a year"), Finnish uses the inessive case (-ssa / -ssä, meaning "in"). You can think of it literally as "a hundred euros in a year". You will see this pattern often with time periods, such as kuukaudessa (a month) or päivässä (a day).

Why is the word euroa in the partitive case here?

In Finnish, any number greater than one requires the noun following it to be in the partitive singular case. Because the amount here is sata (one hundred), the base word euro takes the partitive ending to become euroa.

The amount is the object of the verb maksaa, so why isn't the number sata in a traditional object case?

Verbs expressing a measure, weight, or price—like maksaa (to cost), painaa (to weigh), or kestää (to last)—are an exception to normal object rules. They take their measure amount in the basic nominative form rather than the partitive or genitive. Therefore, the exact price stays in its base form as sata.

I have seen the verb maksaa used to mean "to pay". How do we tell the difference?

The verb maksaa is a very common word that means both "to cost" and "to pay". You can always tell which meaning is intended by looking at the subject. Since an inanimate object like vakuutus (insurance) cannot hand over money, it must mean "costs". If a person were the subject, it would mean "pays".