Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Make the verb agree with its subject, not with a subject complement
Make subjects and verbs agree
Grammar

One basic sentence pattern in English consists of a subject, a linking verb, and a subject complement: Amir is a lawyer. Because the subject complement (lawyer) names or describes the subject (Amir), it is sometimes mistaken for the subject. (See 48b on subject complements.)