Subjects and verbs must AGREE with one another in number (singular or plural). Thus, if a subject is singular, its verb must also be singular; if a subject is plural, its verb must also be plural.
Here are some examples of subject verb agreement (the subject is bolded and the verb underlined):
My dog always growls at the postal carrier
.Basketballs roll across the floor.I don’t understand the assignment.
These clothes are too small for me.
Peter doesn’t like vegetables.