Subject-verb agreement errors occur in the English language when a writer or speaker has mismatched the number (singular or plural) of the verb with the number of the subject of the sentence. It is not always easy to spot a subject-verb disagreement because of the many exceptions to the rules of English, but some general guidelines will help the careful writer find most errors.
The trick to check for subject-verb disagreement is to identify the verb in a sentence. The verb in the sentence will help you find the subject, which will tell you if you have the correct form of the verb. The verb is most easily identified as a word that can come directly after the pronouns "I," "you," "she" and "it." In cases where two words in the sentence could fit after the pronoun, the verb is the word that changes if you change the tense of the sentence. For example, in the sentence "The exhausted runner crossed the finish line," both "exhausted" and "crossed" could come after a pronoun. When we change the sentence from the past tense to the present, "The exhausted runner crosses the finish line," we see that, because "cross" changed to make the sentence present tense, it is the verb.
Once you have identified the verb in a sentence you can identify the subject by asking yourself a question beginning with the words "who" or "what" and following it with the verb. In the sentence "The exhausted runner crossed the finish line," you ask yourself "Who or what crossed?" The runner is the who that crossed, so "runner" is the subject of the sentence. Sometimes the subject of a sentence is more than one word. "The way she won surprised them," is an example. The subject is identified with the same method. The "what" that surprised is "The way she won," the subject of the sentence.
Now that you know the subject of a sentence, you need to identify whether it is singular or plural. There are more exceptions than hard and fast rules in the English language, but there are some guidelines that will help. Most plural nouns are formed with the addition of a terminal "s." When discussing more than one girl, we say "girls." Some nouns take irregular plural forms, such as "man." We do not say "mans" to describe more than one man, we say "men." Other examples include "person" and "people," "mouse" and "mice" and the word "deer," which does not change at all in the plural form. Other subjects are singular but appear plural and take plural verbs, such as "jeans." We say "My jeans are wet," not "My jeans is wet." These are distinctions that are learned through memorization and familiarity with the English language.
Subject-verb agreement errors happen when the writer or speaker uses the plural form of a verb when the subject calls for the singular form, or when the singular form of a verb is used then the subject calls for the plural form. "The subject disagree with the verb," is an example of a subject-verb agreement error. You would say "The subject disagrees with the verb." Use your verb to find your subject, then make sure you have used the right verb for the subject to find subject-verb agreement errors.