Learn Finnish: Minä autan hänen isoäitiään.

In Finnish, for the third person ("his", "her", or "their"), you generally must use both the genitive pronoun (hänen or heidän) and the possessive suffix on the noun. Unlike with "my" or "your", where you can often drop the pronoun, you cannot drop hänen here.

Why is the noun in the partitive case (isoäitiä) before adding the suffix?

The verb auttaa (to help) always requires its object to be in the partitive case. Even when you add a possessive suffix to a word, the core word must still take the grammatical case that the verb demands.

How exactly do we get from isoäiti to isoäitiään?

First, we put isoäiti into the partitive case because of the verb, making it isoäitiä. Then, we add the third-person possessive suffix. Here, we use the -Vn variant, which means we double the last vowel and add n. So, isoäitiä + än becomes isoäitiään.

Could I use the -nsä suffix instead of doubling the vowel?