What did Scrooge learn?

Scrooge learned that the "shadows" of things to come can be changed.... that he had another chance to make things right.

"They are not torn down!" cried Scrooge, folding one of his bed-curtains in his arms, "they are not torn down, rings and all. They are here -- I am here -- the shadows of the things that would have been, may be dispelled. They will be! I know they will."