The single most difficult notion for graduate students and new professors to grasp about teaching--and, indeed, many experienced teachers never grasp this point either--is that successful teaching to undergraduates has little to do with the degree of one's mastery of disciplinary knowledge.
Gregory, Marshall W., "Do We Teach Disciplines or Do We Teach Students?—What Difference Does It Make?" (2008). Scholarship and Professional Work - LAS. Paper 140.