Together with the infinitive they serve as predicative markers and express relational meanings of the subject attitude type (ability, obligation, permission, advisability, etc.). They also express relational probability, serving as probability predicators.
The modal verbs can, may, must, shall, will, ought, need, used (to), dare are defective in forms, and are suppletively supplemented by stative groups (be able to etc). The verbs be and have in the modal meanings ‘be planned’, ‘be obliged’ and the like are also included in the general modal verb list.