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The concept of number applies to nouns, verbs, and pronouns. Number distinguishes “oneness” and “more-than-oneness,” that is, it distinguishes the singular from the plural.

English verbs do not have a special form to denote plurality. Instead, number shows up only in the third-person singular, where the endings of ‑s, ‑es, or -ies indicate the singular.

The personal pronouns also exhibit number. Each of the three persons has both a singular form and a plural form (for second person you, it's the same). In the first person, I (singular) and we (plural). In the second person, you (singular) and you (plural). And in the third person, he or she or it (singular) and they (plural).

Here's a cardinal rule of grammar: A verb must agree with the number of its subject. Another rule: Only the grammatical subject, not some other word, determines the number of the verb.

Here's another cardinal rule of grammar (although it's currently undergoing a change): A pronoun must agree with the number of its antecedent; if the antecedent is singular, the pronoun must be singular; if the antecedent is plural, the pronoun must be plural.

Problems of subject-verb disagreement in number and pronoun-antecedent disagreement in number are thoroughly discussed in the sections on Nouns, Verbs, and Pronouns in Parts of Speech on Grammar.com.

Also, please read the first chapter in the section on Common Grammatical Mistakes. We urge you to study these problems carefully. Click here for the beginning of that discussion. You may also download the eBook Common Grammatical Mistakes.

Nothing in grammar contaminates your style more than violation of these principles.