Modals are a peculiar kind of verbs since they work in a different way from the rest of the verbs. This verbs do not need auxiliary verbs because they can work as auxiliaries, do not have inflection in the third person singular of the present simple, and they are followed by infinitives without ‘to’ among others. Here you have some links where you can practice with modal auxiliaries. It is not difficult once you have clear their uses. I hope it is useful!!