In any complete English sentence, there must be a subject and a verb.
Subject-verb agreement, then, is all about numbers. Does the verb form chosen match up with the number of things in the subject?
This being English, there are of course many exceptions to the rule. However, two in particular are quite common.
Example
E.g. John and Todd are downstairs. The word "and" joins the two subjects, so we use the plural form of the verb.
Examples
Either John or Todd is downstairs. "Todd" is closest to the verb, so the verb agrees with the singular "Todd."
Either John or the twins are downstairs. "The twins is closest to the verb, so the verb agrees with the plural "the twins."
While there are additional exceptions, this covers the basics. Following these steps will help ensure that you always have subject-verb agreement.
Source: Dan Reade
Any complete English sentence must contain both a subject and a verb, so making sure that the subject and the verb agree with each other is critical. The video below provides a basic definition of subject-verb agreement. It also explains several more complicated instances of subject-verb agreement.
Source: Youtube
Subject-verb agreement is one of the most basic parts of English, yet it contains all sorts of pitfalls for the writer. Reviewing the materials above will help you more fully understand subject-verb agreement and avoid many of the common mistakes that can occur in writing.
Source: Dan Reade