All modal verbs are auxiliary verbs, which means they can only be used with a main verb. Modal verbs cannot be a main verb. They are used to express ideas such as possibility, prediction, speculation, deduction and necessity.
12 verb tenses English grammar - 12 Verb Tenses that are in the English language. Tenses are all used to indicate action that has taken place in the past, present, and future. Learning English vocabulary and grammar