In general passive with -s describes an abstract proces - time and place unregarded - whereas passive with blive describes an event in time and place: "happen that...". The modal verbs all have more than one meaning and tend to have one meaning in combination with -s passive and another in combination with blive passive. The exercises forcus on some of the more common meanings, but do not cover all.

Må....-s describes a general permission/prohibition:
Telefonen må gerne benyttes til private opkald. (The phone may be used for private calls) Toilettet må ikke benyttes, mens toget holder. (It's not allowed to use the toilet while the train is in the station)

Må blive describes a dispensation or occasional permission:
Jeg skrev et brev, hvor jeg bad om, at jeg måtte blive inviteret med til kvindemødet, selvom jeg var en mand. (I wrote a letter to ask if I may be invited to the women's lib meeting in spite of beeing a man)
Jeg vil gerne bede om, at gaven ikke må blive åbnet før juleaften. (I'll ask you not to open the present before Christmas)

Choose the right combination in the drop-down list

Kunstværket er personligt og må ikke blive udstilletudstilles offentligt.

Jeg vil gerne bede om, at jeg må blive udstilletudstilles som skulptur på Museet under titlen 'Ekshibitionist'