While nouns refer to specific persons, places, or things, personal pronouns also refer to specific persons, places, or things. Pronouns have characteristics called number, person, and case.
In English, the number refers to the plurality and singularity of the noun. For example, “her” is singular, or “them” is plural. Thus Mary becomes she or her, while Mary’s friends become they or them.
Person is a little more abstract. When we say the first person, we normally use the pronoun I or we – the person speaking. For example, “I visited China last year” is in the first person.
The second person is the one being spoken to–you. For example, “You must take these papers to John as soon as possible.”
The third person is being spoken of-he, she, it, they, them. Consider this sentence: “He visited Thailand last year.”
N.B. It is very important to make sure that the pronoun matches (agree with) its antecedent in person as well as number.
Case, on the other hand, refers to what job a pronoun can legally perform in a sentence. Some pronouns can be subjects and others cannot. For instance, it’s okay to say “I expect to finish this course soon,” but we are not allowed to say “Me expect to finish this course soon.”
Pronouns that may be subjects are in the subjective case; they are subject pronouns. However, some pronouns cannot be subjects; rather, they are used as direct objects, indirect objects, or objects of prepositions. They are in the objective case; they are object pronouns: “Her aunt employed her after graduation.” “Miss Mary gave her a job, too.” “Without them, she would have been unemployed.”