"The Godfather, Part II" moves both forward and backward in time from the events in "The Godfather," in an attempt to resolve our feelings about the Corleones. In doing so, it provides for itself a structural weakness from which the film never recovers, but it does something even more disappointing: It reveals a certain simplicity in Coppola's notions of motivation and characterization that wasn't there in the elegant masterpiece of his earlier film.