Learn Finnish: Silmätipat auttoivat molempia silmiä eilen. - The eye drops helped both eyes yesterday.

The verb auttaa (to help) is a partitive verb, which means the person or thing being helped must be in the partitive case. Because there is more than one eye involved, both the pronoun molemmat and the noun silmä must be put into the partitive plural, becoming molempia silmiä.

How do we get the form molempia from the base word molemmat?

The word molemmat (both) behaves uniquely. When you add case endings, the stem changes to either molempa- or molempi-. To build the partitive plural, we use the -mpi- stem and attach the partitive ending -a, resulting in molempia.

How is the partitive plural silmiä formed from silmä?

For short nouns ending in -ä, the final vowel drops out to make room for the plural marker -i-. After removing the -ä and adding the -i-, we attach the partitive ending -ä to the end. So, silmä turns into the stem silm-, plus -i-, plus -ä, creating silmiä.

Let us review the past tense verb auttoivat. Why did the -a- turn into an -o-?