Three Sounds of "ed" Past Tense Verbs - be sure that students learn this- that walked is not spelled "walkt", etc. This leads to a "for purchase" unit... am just including it in case this might be helpful.
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.