Answer:
The yew tree (the monster) taught Conor through its stories is that Conor should accept and understand that his feelings regarding his mother's sickness are contradictory, that he wants to let his mother go even while desperately wanting to save her from death, and that these feelings do not necessarily make him a bad person.
And also that only by acknowledging that fact can Conor be free of the guilt he feels.
In essence the monster helps Conor interpret his feelings and understand that they are just as valid as those of the characters in the monster's stories.
Explanation:
Conor expects all the stories the monster tells him to bear clear moral lessons because of the similarities they bear to his own life, but the monster implies that it was only trying to help him understand human nature and life itself.
However despite this the monster's stories are supposed to make Conor able to see his life from a different perspective and interpret his contradictory emotions, which are all lessons in reality.
Conor learned that the primary lessons of all three stories in the monster's words, "There is not always a good guy. Nor is there always a bad guy. Most people are somewhere in between."
