Pronouns
A pronoun is to agree with its antecedent in number (singular/plural) and gender (masculine, feminine, or neuter).
See the following resources:
Hacker, Diana. A Writer's Reference. 6th ed. Boston: Bedford, 2007.
pp. 187-192
G3-a "Make pronouns and antecedents agree."
G3-b "Make pronoun references clear."
Links:
http://www.methodist.edu/english/dd_pref.htm
http://www.methodist.edu/english/pref.pdf
http://www.methodist.edu/english/pr_answers.pdf
For examples of errors and corrections in pronoun reference, see the attached PowerPoint presentation.