The answer is the problems of
the human heart. Because Faulkner believes that is the only thing that is worth
writing about and worth the agony and the sweat. He also said that a man
or woman must learn them again, in order to write from the heart.
I'm not sure what the lesson is, but the answer that makes sense the most is number. Number because most lessons in subject-verb agreement concerns the singularity and plurality of the subject at hand
