Among the most common grammar mistakes that occur in English is subject-verb agreement. Because sentence structure is fundamental in ensuring a sentence makes sense in English, getting this agreement right is paramount. Even if readers get what you mean, lack of agreement can make you appear uneducated.
Subject-verb agreement starts with knowing whether your sentence is about one or more than one object. If it’s one person, place or object as in The parent is very upset, then your subject (parent) is singular. If it’s more than one as in The parents are very upset, then your subject (parents) is plural.
Next, the verb – which tells what the subject does – needs to match the subject. If you use a singular subject, then you must use a singular verb. For example, in The customer is very happy, both the subject (customer) and the verb (is) are singular. If you use a plural subject, then you must use a singular verb. To wit, in The customers are very happy, both the subject (customers) and the verb (are) are plural.
Usually agreement problems become an issue when the writer is confused as to whether a subject is singular or plural. For example, does everyone refer to a single body of people and so is singular or does it refer to several people and so is plural? If you’re uncertain, a simple solution to get the right answer is to replace the subject with a pronoun and then use it in a sentence with first a singular and then a plural verb. Everyone is present thus could become They is present or They are present. Replace the original sentence with the replacement sentence that is correct, so you could instead write They are present.