Learn Finnish: Miksi vapaaehtoiset eivät keränneet rahaa? - Why didn't the volunteers collect money?

The verb is kerätä (to collect), which is a type 4 verb. To form the past participle for type 4 verbs, you take the infinitive stem (kerä-) and add -nneet for the plural. The double "n" is characteristic of type 4, 5, and 6 verbs in this form!

How does the negative past tense work here with eivät keränneet?

In Finnish, the negative past tense is formed by using the conjugated negative verb (eivät for "they") alongside the past participle form of the main verb. Because the subject (vapaaehtoiset) is plural, the main verb takes the plural past participle ending -neet.

How do we get vapaaehtoiset from the base word for "volunteer"?

The dictionary form is vapaaehtoinen. For Finnish words ending in -nen, you must change that ending to -se- before adding the plural marker -t. This gives us vapaaehtoiset.

Why is rahaa (money) in the partitive case?

There are two rules pointing to the partitive here! First, in negative sentences, the object is almost always in the partitive case. Second, raha (money) is an uncountable mass noun in this context (they are collecting an undefined amount of money), which also triggers the partitive.