We have no free choice over the verb to use when we construct a sentence. We have to look at the subject for it decides the verb that we can use. There are rules that govern the use of the verb in relation to the subject in a sentence. The subject verb agreement calls for both the subject and verb to agree with each other in person, number, and more. For example, a singular subject takes a singular verb and a plural subject takes a plural verb. There are however exceptions to this agrement.
In the present tense, the rule of agreement states that the verb must have an added –s if the subject/noun is singular or a third person singular. The verb does not have an –s if the noun is plural. This means verbs have singular and plural forms only in the present tense (eat/eats, sit/sits). In the past tense, only the auxiliary verbs (was/were) change but not the main verbs with the person and number of the subject/noun.