For a sentence to be grammatically correct, the verb must be in consistent agreement with its subject in singular or plural number. Most, but not all, singular subjects require a singular verb which ends in “s.” Likewise, most plural subjects abandon the “s” in the verb. The following examples will help demonstrate subject-verb consistency with singular and plural forms.
 
Example 1: A healthy diet reduce the risk of high blood pressure.
In this example, there is a singular subject (“diet”). Therefore, the verb (“reduce”) must either be changed to the “s” form in order to agree with the singular subject, or the subject must be pluralized to agree with the current plural-form verb.
Solution 1: A healthy diet reduces the risk of high blood pressure.
Solution 2: Healthy diets reduce the risk of high blood pressure.
 
Additionally, linking verbs (forms of the verb, “to be”) must have agreement with their corresponding subjects.
 
Example 2: These four pieces of evidence is the basis of her argument.
Here, “pieces of evidence” is plural, so the linking verb “is” must be changed to the plural.
Solution 2: These four pieces of evidence are the basis of her argument. 
 
 
 
 
 
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