Pronouns in the objective case may function as

subjects
objects
adjectives
predicate nominatives

Pronouns in the objective case may function as objects. When it comes to the parts of speech used in the objective case, the only function they can have is that of the object in a sentence. That object can be a direct object, an indirect object, or the object of a preposition. These pronouns thus cannot be subjects, because then they would be in the subjective case. Pronouns cannot be adjectives, and predicate nominatives are only nouns, which pronouns are not.