You use be with an infinitive to say or ask what should happen or be done in a particular situation, how it should happen, or who should do it.
后接动词不定式,用于说明或询问特定情况下应该会怎样、怎样做、谁来做等
You use be with an infinitive to indicate that something is planned to happen, that it will definitely happen, or that it must happen.
后接动词不定式,表“安排、注定、必然”等意