Subject-Verb Agreement
A verb is to "agree" with its subject in number (singular or plural).
See the following resources:
Hacker, Diana. A Writer's Reference. 6th ed. Boston: Bedford, 2007.
pp. 163-171
G1 "Subject-verb agreement."
Harris, Muriel. Prentice Hall Reference Guide. 6th ed. Upper Saddle River, NJ: Pearson, 2008.
pp. 110-117
11 "Subject-Verb Agreement"