It is most common when a person is grammatically incorrect when it comes to the verb agreeing with its subject. This is known as the Agreement of Subject and Verb.
A verb should agree in number with its subject. A word that refers to one person, place, thing, or idea is singular in number. A word that refers to more than one is plural in number.
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In a verb phrase, the first helping verb agrees in number with the subject.
Example: Ashley is attending college.
They are attending college.
Singular subjects take singular verbs and Plural subjects take plural verbs. The number of a subject usually is not determined by a word in a phrase or clause following the subject.
A gerund phrase or an infinitive phrase used as a complete subject takes a singular verb. Do not be misled by any particular noun or pronoun in the phrase. The gerund or infinitive serves as a singular simple subject.
A compound subject consists of two or more subjects that are joined by a conjunction and that have the same verb.
Subjects joined by and generally take a plural verb.