“Agreement: The correspondence of a verb with its subject in person and number (Karen goes to Cal Tech; her sisters go to UCLA), and of a pronoun with its antecedent in person, number and gender (As soon as Karen finished the exam, she picked up her bookbag and left the room).”
Excerpted from: Strunk, William Jr., and E.B. White. The Elements of Style, Fourth Edition. New York: Longman, 2000.