Complex sentence: In 1991, Tolbert assumed the position of chair of Howard University's History department, holding the post until 1998, then again from 2002 to 2005, and 2009.
Split into simpler sentences: In 1991, Tolbert assumed the position of chair of Howard University's History department. He held this post until 1998, then served as chair again from 2002 to 2005, and 2009.