In this article, we will focus on what we call "subject-verb agreement." In English, the rule is that the subject of the sentence (what the sentence is about) always has to agree with the verb (the action of the sentence). In simple terms, this means that there is a specific form of a verb for every subject, the subjects being I, you, he, she, it, we, they. When the subject and the verb do not agree, this is a very noticeable grammar error and should be avoided at all costs.
The most common error is made with what we call the third person singular form of a verb in the present tense. The third person singular form refers to "he", "she" and "it." Many of the errors occur with the verb "to do" (do, does, did). The verb that goes with this form always has to have an "s" at the end, so for the verb "to do", the correct form would be "does."
Let's look at an example:
This sentence is correct: "She doesn't know them." The pronoun "she" is the subject and the helping verb "doesn't" (does not) is the correct form that goes or agrees with it. The form "does" agrees with "she" because it has an "s" at the end of it, as third person singular forms must have.
Many people, however, do not use the correct form of the verb when using the third person singular present tense..
This sentence is not correct: "She don't (do not) know them." The pronoun "she" is the subject, but this time the helping verb "do" is used, which does not have an "s" at the end of it. The form "do" is used with all subjects, including "I", "you", "we" and "they." The only one it is never used with is the third person singular form, including "he", "she" and 'it."
In summary, there are minor grammar errors that most of us notice but overlook. Because so many people make them, they are becoming more acceptable in informal everyday conversational speech. Then, there are some major grammar errors that are very noticeable and may stereotype us as to our educational or socio-economic level. Forming the third person singular present tense verb incorrectly certainly gives our listener a negative impression and may even be a deciding factor in obtaining a job in which public speaking is important.