Complex sentence: Bart takes Cathy and Corrine to the library--where Grandmother Olivia is seated--at first thinking that Cathy is lying, but after hearing Cathy's whole story, he confronts Corrine.
Split into simpler sentences: Bart takes Cathy and Corrine to the library where Grandmother Olivia is seated. At first, Bart believes Cathy is lying, but after hearing Cathy's whole story, he confronts Corrine.