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Subject-Verb Agreement: Basic Principles

In standard English, the rule is that with subjects and verbs, and with quantity words and the nouns they modify, singular is matched with singular form and plural is matched with plural form.

Within any clause with a present tense verb, the subject and verb must agree in person (first, second, or third) and number (singular or plural). The ending -s is added to nouns and verbs in English, but in very different contexts.
  1. An -s ending on a noun is a plural signal: her brothers (more than one).
  2. An -s ending on a verb is a singular signal; -s is added to a third person singular verb in the present tense: Her mother wears gold jewelry. The third person subject of the verb determines the form of the verb: plural subject, plural verb form (no -s); singular subject, singular verb form (with  -s).
In standard English, a third person singular subject must have a singular verb (with -s), and a plural subject must have a plural verb (with no -s).
  A baby cries. Babies cry.
  He loses. They lose.
  His brother plays baseball. His brothers play baseball.
When a sentence contains a series of verbs, maintain consistency of form with every verb in the series:
  Example She uses her experience, speaks to the crowd, and wins their confidence.
Most simple present verbs show agreement by an -s ending; however, the verb be has four instead of two present tense forms. In addition, be is the only verb to show agreement in the past tense, where it has two forms: were and the third person singular form was.
Subject-Verb Agreement
Simple Present Base Forms  
like have be do
  1st person: I like have am do
  2nd person: you like have are do
  3rd person: he, she, it likes has is does
  1st person: we like have are do
  2nd person: you like have are do
  3rd person: they like have are do

Two Key Points about Agreement
  1. Follow the one -s rule. You can either put an -s on the noun to make it plural or put an -s on the verb to make it singular. An -s on both subject and verb is not standard English.
  Faulty    My friends comes over every Saturday.
  Revised    My friend comes over every Saturday.

My friends come over every Saturday.
  1. Do not drop a necessary -s.
  Examples His mother wants him to finish school.

The books on my desk describe life in Tahiti.
Note: Modal verbs never add an -s ending, and they are always followed by a base form: I can sing; she should go; he might leave; she will try; they must speak.
See also
Sentence Problems: Subject-Verb Agreement