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.)

The subjects neighbor and sulfur dioxide are third-person singular, so the verbs must end in -s.

tip: Do not add the -s ending to the verb if the subject is not third-person singular. The writers of the following sentences, knowing they sometimes dropped -s endings from verbs, overcorrected by adding the endings where they don’t belong.

The writer mistakenly concluded that the -s ending belongs on present-tense verbs used with all singular subjects, not just third-person singular subjects. The pronoun I is first-person singular, so its verb does not require the -s.

The writer mistakenly thought that the verb needed an -s ending because of the plural subject. But the -s ending is used only on present-tense verbs with third-person singular subjects.

Has versus have

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

The subject, musician, is third-person singular, so the verb should be has.

The subject, classes, is third-person plural, so Standard English requires the verb have. Has is used only with third-person singular subjects.

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.

Grandfather is third-person singular, so the verb should be doesn’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).

The subject you is second-person singular, so the verb should be were.