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.
Choosing among the words "during," "while," and "for" can sometimes be confusing. Check out this American English at State graphic to learn how to correctly use each of these three words. What other pair or set of words are sometimes confusing to you?