Luke 9:45

It was concealed from them

(ην παρακεκαλυμμενον απ' αυτων). Periphrastic past perfect of παρακαλυπτω, a common verb, but only here in the N.T., to cover up, to hide from. This item only in Luke.That they should not perceive it

(ινα μη αισθωντα αυτο). Second aorist middle subjunctive of the common verb αισθανομα used with ινα μη, negative purpose. This explanation at least relieves the disciples to some extent of full responsibility for their ignorance about the death of Jesus as Mark 9:32 observes, as does Luke here that they were afraid to ask him. Plummer says, "They were not allowed to understand the saying then, in order that they might remember it afterwards, and see that Jesus had met His sufferings with full knowledge and free will." Perhaps also, if they had fully understood, they might have lacked courage to hold on to the end. But it is a hard problem.