Jill: If you "could have", why didn't you?
Jack: Something stopped me.
Jill: That means you "couldn't have" done it even if you wanted to.
Jack: I "could have" but there were to many obstacles then.
Jill: It's because of those obstacles that you "couldn't have" done it.
Jack: If I knew then what I know now I "could have" done it.
If you were back there now, you still
wouldn't know what you know now,
so you still "couldn't have" done it.
Jack: I think... I just broke my crown.