Subject-verb agreement means that the subject and the verb of a sentence must agree in number, meaning both need to be singular or both need to be plural. Watch out for collective nouns: words such as team, university, group, and family are generally all singular even though they are made up of more than one person.
Important to know! When the ACT tests subject-verb agreement, it will often put a lot of words between the subject and verb to fool your ear. For example: “The philosophy of organic architecture, with the scope of its meaning mirrored in the developments of Wright’s various architectural projects, was/were consistently present in many of his works and developments.” The subject is philosophy, which is singular, so the answer is “was.”