Breakdown of Samvinnan milli þeirra er frábær af því að þeir styðja hvor annan.
vera
to be
þeir
they (m)
þeirra
their / theirs
af því að
because
Questions & Answers about Samvinnan milli þeirra er frábær af því að þeir styðja hvor annan.
Why is the phrase for each other split into two different cases here, with hvor in the nominative and annan in the accusative?
This is called split case government. The first part (hvor) acts as the subject of an implied action, so it stays in the nominative case. The second part (annan) is the object of the verb styðja (to support), which requires the accusative case. It translates literally to something like 'each [supports] the other.'
Why does the sentence use hvor annan instead of hver annan?
Hvor is used when referring to exactly two subjects, while hver is used for three or more. Because the sentence uses hvor, we know for a fact that exactly two people are cooperating. If it were a group of three or more men, it would be þeir styðja hver annan.
How does the grammar show that this sentence is specifically about two males?
The pronoun þeir is the masculine plural for 'they.' Furthermore, the reciprocal pronoun matches this gender: hvor and annan are both masculine. If it were two women, you would see þær styðja hvor aðra, and for a mixed-gender pair, it would be þau styðja hvort annað.
Why is þeirra used after milli instead of an accusative or dative pronoun?
The preposition milli (between) always governs the genitive case. Þeirra is the genitive plural form of the pronoun þeir.