What metaphors are used in We Live by What We See at Night by Martin Espada?

In Lines 7-10, it's difficult to tell if this is a real bridge that the grandfather built, or just a figurative one. Often, the image of "crossing a bridge" is used as a metaphor to express a passage from one world to another, to express a personal growth and progression. In this case, since in the ninth and tenth lines readers learn the bridge is built over a river seen only in a dream, they might guess this is not a real bridge.