Subjects and verbs must “agree” in number (singular or plural) and person. The concept of agreement is particularly relevant to:
1) the present simple tense: subjects in 3rd person singular (he, she, it) take verbs with the ending -s
2) the verb to be: it can take forms am/is/are and was/were
3) the verb to have: it can take forms have/has
Note: If a sentence contains a modal verb (e.g. should, can, could, must, may), this modal must be followed by the base form of the verb regardless of the subject’s number and person. For example: The witness must report to the police (not must reports).
Below are the rules for subject-verb agreement.
Make sure the verb agrees with its subject, not with a word that comes between the subject and the verb (whether in preposition phrases or adjective clauses).
For example: The samples on the tray in the lab need testing. (prepositional phrases)
The governor, as well as his press secretary, was applauded. (an adjective clause) Note: A subject can never be a part of a prepositional phrase (e.g. in the lab is a prepositional phrase because it starts with the preposition in, so the noun lab cannot be a subject of a sentence).
Make the verb agree with its subject even when the subject comes after the verb.
For example: There are surprisingly few children in our neighborhood.
Treat subjects joined with and as plural.
For example: Matt and Lisa often write in the morning.
Sonja’s ability and desire to help are inspiring.
However, when the parts of the subject form a single unit or when they refer to the same person or thing, treat the subject as singular. In addition, when a compound subject is preceded by each or every, treat the subject as singular.
For example: Strawberries and cream was a last-minute addition to the menu.
Each tree, shrub, and vine needs to be sprayed.
With subjects connected by or or nor (or by either ... or or neither ... nor), make the verb agree with the part of the subject nearer to the verb.
For example: A driver’s license or credit card is required.
Treat most indefinite pronouns as singular. Indefinite pronouns refer to nonspecific persons or things, and include the following: anybody, anyone, each, either, everybody, everyone, everything, neither, nobody, none, no one, somebody, someone, and something.
For example: Everyone in the class likes the teacher.
Treat collective nouns as singular unless the meaning is clearly plural.
For example: The class respects the teacher. (singular)
The classes are debating among themselves. (plural)
Titles of works, company names, words mentioned as terms, and gerund phrases are singular.
For example: Lost Cities describes the discoveries of many ancient civilizations. (title)
“Controlled substances” is a euphemism for illegal drugs. (term)
Encountering busy signals is troublesome to many people. (gerund phrase)
Adapted from: A Writer’s Reference (7th ed.) by Diana Hacker.