A pronoun is a word that replaces a noun. The noun that has been replaced
is referred to as the antecedent for that pronoun. Pronouns must agree with
their antecedents in number. A singular antecedent needs a singular pronoun
and a plural antecedent needs a plural pronoun. (来源：英语学习门户网站EnglishCN.com)

Indefinite pronouns are singular and must be replaced with singular
pronouns, not plural. Using plural pronouns with indefinite pronouns is a
common mistake. Because indefinite pronouns are singular and require singular
pronouns as replacements, the indefinite pronouns often create a problem with
sexist language. To avoid the problem writers often rewrite their sentences to
use plural antecedents so they can use plural pronouns. In most cases,
collective pronouns are also singular and require singular pronouns.

(来源：英语麦当劳www.EnglishCN.com)

Pronouns require a clear antecedent to refer to. Writing that leaves the
antecedent for a pronoun vague is often confusing and difficult to follow. It
can also result in unintentional comedy on the part of the writer when the
antecedents for the unclear pronouns are misinterpreted. It is crucial during
the editing process for you to check each pronoun and connect it to its
antecedent to be certain that the antecedents are clear and that there is
agreement between the antecedent and the pronoun.

Chapter 15: Using Pronouns Correctly: Consistency and Case

Consistency in pronoun use refers to the point of view you are writing from.
There are three points of view: first person, second person, and third person.
First person is I and We; second person is you; third person is he, she, it, and
they. To be consistent you must write in the same person throughout your
paragraph unless there is a very good reason to switch person. (来源：英语学习门户网站EnglishCN.com)

Case refers to using the right pronoun for the right job. The three main
cases for pronouns are the subjective case, the objective cases, and the
possessive case. You must use the appropriate pronoun for the function the
pronoun has in the sentence. Objective pronouns like me cannot be used as the
subject of a sentence, and subject pronouns like I cannot be used as the objects
of prepositions or as direct/indirect objects in a sentence. Always check your
sentences carefully to be certain you have used the correct pronoun in the
sentence for the function you have given the pronoun. (来源：英语麦当劳－英语杂志 EnglishCN.com)

Chapter 16: Punctuation