Complex sentence: In the summer of 2000, Trachtenberg took on the role of Dawn Summers in "Buffy the Vampire Slayer"; she remained in the role until the show ended in 2003.
Split into simpler sentences: In the summer of 2000, Trachtenberg took up the role of Dawn Summers in "Buffy the Vampire Slayer". She remained in the role until the show ended in 2003.