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 G1.)

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.