Nouns and verbs must agree in number. A singular noun must be used with a singular verb.
- My sister is an auditor. (Single noun sister agrees with single verb is.)
- The car is parked at the used car lot. (Single noun car agrees with single verb is parked.)
Likewise, a plural noun must be used with a plural verb for there to be agreement.
- My sisters are accountants. (Plural noun sisters agrees with plural verb are.)
- The cars are parked at the used car lot. (Plural noun cars agrees with plural verb are parked.)