Learn Finnish: Minä ostan pari omenaa. - I am buying a couple of apples.

In Finnish, after numbers greater than one and quantity words like pari (a couple), you must use the partitive singular form of the noun. So instead of a plural word, Finnish uses the singular partitive omenaa.

Is pari a number? Why does it trigger the partitive case here?

The word pari means a pair or a couple. Even though it isn't a strict number like kaksi (two), it acts as a quantifier in this sentence. Just like numbers from two upwards, it forces the noun that follows it into the partitive case.

How do we form the partitive omenaa from the basic dictionary word for apple?

The basic dictionary form for apple is omena. Because it ends in a single vowel (-a), you simply add another -a to the end to make the partitive form omenaa.

Does pari mean exactly two apples, or just a few?

Just like the phrase "a couple" in English, pari can mean exactly two (a pair), but in everyday speech it is often used loosely to mean two or three.