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

Use -s or -es endings on present-tense verbs that have third-person singular subjects
Choose appropriate verb forms, tenses, and moods
Grammar

All singular nouns (child, tree) and the pronouns he, she, and it are third-person singular; indefinite pronouns such as everyone and neither are also third-person singular. When the subject of a sentence is third-person singular, its verb takes an -s or -es ending in the present tense. (See also 21.)

NOTE: Do not add the -s ending to the verb if the subject is not third-person singular. The writers of the following sentences added —s endings where they don’t belong.

Has versus have

In the present tense, use has with third-person singular subjects; all other subjects require have.

Does versus do and doesn’t versus don’t

In the present tense, use does and doesn’t with third-person singular subjects; all other subjects require do and don’t.

Am, is, and are; was and were

The verb be has three forms in the present tense (am, is, are) and two in the past tense (was, were).