Subject-verb agreement is a rule stating that all subjects must agree with their verbs in number. Singular subjects take singular verbs. Plural subjects take plural verbs.
Most of the subject-verb agreement questions on the SAT deal with verb forms in the third-person singular form (he/she/it/one) and third-person plural forms (they).
In the present and present perfect verb tenses, third person singular verb forms end in an “s”. Third person plural verb forms do not.