Questions & Answers about He pitävät toisistaan ja jakavat uusia muistoja.
How is the word toisistaan ("each other") formed in this sentence?
The base word is toinen (other). To say "each other", Finnish uses the plural form of toinen in the correct case, followed by a possessive suffix. Here, the plural elative form is toisista, and the third-person possessive suffix is -an (matching He, meaning "they"). Together, they make toisistaan.
Why is toisistaan in the elative (-sta) case here?
The verb pitää (when it means "to like") always requires the thing being liked to be in the elative case. That is why we use the elative form toisista- instead of the partitive form toisia- (which you would use with a verb like rakastaa, meaning "to love").
Why are uusia and muistoja in the partitive plural case?
They are the object of the verb jakavat (share). When a plural object is an undefined amount (they are sharing "new memories", not a specific number or "the new memories"), it must be in the partitive plural case.
I know uusi means "new", but why does it take the form uusia here?