jezeli modal odnosi sie do jakiejs czynnosci, to tak, jak ma to miejsce w przypadku czasow ciaglych, czasownik, plus modal z forma 'be', wystepuje w formie ciaglej ('modal + be' pozostaje zawsze taka sama, bez wzgledu na osoby):
Kieran is reading in his bedroom.
Kieran MUST BE reading in his bedroom.
Kieran MAY BE reading in his bedroom.
Sally and her sister aren't going to the seaside.
Sally and her sister CAN'T BE going to the seaside.
Sally and her sister MAY NOT BE going to the seaside.