Question

When the act first opens, explain why bernardo is on the edge?

Answer 1

When the act first opens, Bernardo appears to be on the edge because he is much worried to run into the ghost.  Bernardo becomes the first to challenge Francisco because Bernardo had seen the ghost, along with Marcellus, while Francisco had not. Francisco notes that Bernardo should come "carefully" on his hour which means that he should not only come "on time" but also  "cautiously."

Answer 2

This must be the first act on the hamlet I supposed. Francisco is standing on the guard platform which is depicted as being very high on the walls of the castle, with a view of the dark sea. In the book Bernardo, who is to take Francisco's place, comes up onto the platform and looks around. so that's why he's on the edge.