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.
English Grammar solution: Structure of All Tense, Structure of the Tense. i would probably break this chart down into different pieces but most kids have trouble when it comes to the different tenses. so this would help a lot in the classroom.