A pronoun is a word that takes the place of a noun. There are three types of pronouns: subject (he); object (him); or possessive (his). There are twelve rules to pronouns and their uses.
Rule 1. Subject pronouns are used when the pronoun is the subject of the sentence.
Rule 2. Subject pronouns are also used if they rename the subject. They will follow to be verbs.
Rule 3. When referring to a personal pronoun, it takes the verb that agrees with that pronoun.
Rule 4. Object pronouns are used everywhere else beyond Rules 1 and 2.
Rule 5. The pronouns who, that, and which become singular or plural depending on the subject. If the subject is singular, use a singular verb. If it is plural, use a plural verb.
Rule 6. Pronouns that are singular require singular verbs.
Rule 7. To decide whether to use the subject or object pronoun after the words than or as, mentally complete the sentence. When using I or me, it can be interpreted it in two ways: to me OR to that than I.
Rule 8. The possessive pronouns yours, his, hers, its, ours, theirs, and whose never need apostrophes.
Rule 9. The only time it's has an apostrophe is when it is a contraction for it is or it has. The only time who's has an apostrophe is when it means who is or who has. There is no apostrophe in oneself. Avoid one's self, a common error.
Rule 10. Pronouns that end in -self or -selves are called reflexive pronouns. Reflexive pronouns are used when both the subject and the object of a verb are the same person or thing. If the object of a preposition refers to a previous noun or pronoun, use a reflexive pronoun. Reflexive pronouns are also used for emphasis.
Rule 11a. Avoid they and their with singular pronouns. If the gender is undetermined, you could say it with both genders.
Rule 11b. Singular pronouns must stay singular throughout the sentence. The problem is that someone is singular, but they is plural. If they is changed to he or she, it gets a rather clumsy sentence, even if it is technically correct.
Rule 12. When a pronoun is linked with a noun by and, mentally remove the and + noun phrase to avoid trouble.