SARAH: Hamlet doesn't mean "remember" in a literal sense here; how could he possibly forget this encounter with a ghost? What he means is that he's going to keep the Ghost's command for revenge in the forefront of his thoughts, so he will act on it when the moment is right. This is what he is swearing to do.

RALPH: That's right, Sarah. And yet, one of the questions we'll have to ask as we move forward is, does Hamlet keep his promise? Does he act on the ghost's command as promptly as he can, or does he delay?