Question: what instructors teach courses? | Do you mean the first names of instructors who teach courses? | Yes, the first names of all instructors who teach courses,
SQL: SELECT T2.emp_fname FROM CLASS AS T1 JOIN employee AS T2 ON T1.prof_num   =   T2.emp_num JOIN course AS T3 ON T1.crs_code   =   T3.crs_code JOIN professor AS T4 ON T2.emp_num   =   T4.emp_num