Why do these phrases contain a pronoun case? "Each of the applicants must submit their completed forms to the interviewer." and "My best friend and I are going to Hawaii this summer."

Answer 1

Nominative case pronouns are used as subjects and predicate nominatives -
I, you, he, she, we, they

Objective case pronouns are used as direct and indirect objects and objects of prepositions.
me, you, him, her, us , them

Possessive case pronouns show ownership.
his, hers, our,ours, mine, my, their, theirs.