Pronouns and their antecedents are as much as source of grammar mistakes as the subjects and their verbs. Per grammar rules, the antecedent is the word (noun or pronoun) that a pronoun replaces. As a general principle, pronouns and antecedents should agree in gender, number, and person.
When it comes to the number, a singular antecedent should be followed by a singular pronoun, e.g., "Everybody is looking forward to getting his share of the inheritance." Traditionally, his already refers to both men and women in the group, although a compromise can be reached, in which case this sentence can be written as "Everybody is looking forward to getting his or her share of the inheritance." But English grammar rules in informal settings can be passed up, and the example above can also be "Everybody is looking forward to getting their share of the inheritance."
Collective nouns as antecedents in a sentence assume singular pronouns if the group acts as a unit. Hence, "The family was deciding on its issues." When this sentence is written as, "The family was deciding on their issues", it creates confusion and doubt as to the number of its antecedent. In this case, the number of the antecedent should be decided and the pronouns should follow accordingly.
As a rule of thumb who substitutes for persons, which for things, and that for both. Indefinite pronouns, such as either or neither, are followed by a pronoun in the third person: "Either of you completed his assignment." While in the sentence, "Joana hated everybody and everyone that remind her of the past", the antecedent nearer to the pronoun determines its number or gender.
Pronouns also have case forms that signal their place and relationship in a clause or sentence. In the book of grammar rules, there should be three: subjective, possessive, and objective. Subjective or nominative pronouns refer to persons or things, possessive or genitive express possession or ownership, and the objective or accusative denotes a person or thing being pointed at.
A subject that is a pronoun is inherently troublesome; nonetheless the nominative case is assumed: "He and I went together." But in informal English, it should suffice for one to just say, "He and me went to together." When after a linking verb, however, the pronoun also takes on the nominative case, such as "It was she walking by the trees yesterday", or "It was they who called on us from the other side of the street."
Who, whom, whoever, and whomever are probably the pronouns that caused most of the confusion among writers. The English grammar rules have been firm that because who and whoever are used as subjects in a sentence; therefore, they are strictly used as predicate pronouns. Whom and whomever, on the other hand, assume roles as objects of verbs and prepositions. "Doubts as to who committed the crime have been erased." Here, who is the subject of "committed the crime", while the entire clause who committed the crime is the object of the preposition to.