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 rule of agreement calls for the matching of the subject and verb: singular subjects go with singular verbs; plural subjects go with plural verbs.

In the present tense, the rule of agreement states that the verb must have an added –s if the subject/noun is 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, the verb does not change with the number of the subject/noun:

He walked to school.
They walked to school.