Complex sentence: She initially hates Glenn, believing him to be as incompetent as he looked due to his lack of enthusiasm, however when he actually starts doing his job, she begrudgingly begins to respect Glenn as a highly effective teacher.
Split into simpler sentences: She initially hates Glenn, believing him to be as incompetent as he looked due to his lack of enthusiasm. When he actually starts doing his job, however, she begrudgingly begins to respect Glenn as a highly effective teacher.