Modal verbs, which express a likelihood, ability, permission, request, order etc., usually help main verbs to state the future. It is the reason they are categorised as auxiliary verbs in the context.

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.

After that we picked up can, might, must and will as modal verbs but mostly because I was saving yellow's blueprint, guide, model and mold as templates. I innately know what a modal verb is but ...