How is Ferdinand different from Caliban in his relationship to Miranda? Why does he pledge to keep her honor safe?

It is pretty much stated earlier that Caliban try rape Miranda in the past. Prospero confronted him in Act 2, "In mine own cell/till thou didst seek to violate/The honor of my child." Caliban is not sorry for this. He actually says he wishes he would have succeeded in the rape. He even intones that he would have raped her repeatedly and filled the island with his offspring, "O ho, O ho! Would'st had been done!/Thou didst prevent me; I had peopled else/This isle of Calibans." Calibans feelings are hence much more primal than say Ferdinand's. For your last question Ferdinand promise that his honor will never be melted by lust thus keeping her honour safe.