* "Tutoring students in afternoons is helpful." The subject of the sentence is "tutoring students in afternoons," which is a gerund phrase (a noun phrase starting with a gerund, "tutoring").
* Gerund phrases do not have a pronoun case. Pronoun cases apply to pronouns (words that replace nouns), and the bolded phrase is not a pronoun. It functions as the noun in the sentence.
Therefore, there is no pronoun case to identify in this sentence.