Picture a small child saying: "Can’t, shan’t, won’t!" They are using modal verbs to show their feelings and attitude to something. Maybe they are refusing to go to bed or eat their peas.

what you are supposed to do/what you should do/what you ought to do (It often means 'shall', eg shall/should I help you?) sollen to be supposed to, ought, should what you may do/what you are ...