Subject mastery alone does not make a good teacher. A teacher who knows a subject deeply but cannot explain it, listen to learners, or diagnose a misconception will fail in the classroom. Good teaching combines mastery with communication, planning, patience, fairness, empathy and professional ethics.
Worked example: a teacher notices that one student's answers suddenly worsen. Instead of scolding the student, she talks privately, learns that the student cannot see the board clearly, and moves his seat. The subject knowledge mattered less here than observation, empathy and a practical fix.