Complex sentence: After telling Ethan that he has failed to convince Angela to go out with her, Ethan reveals to Dave he has been obsessing over Angela for quite some time and reminds Dave that he still intends to get Dave and his friends expelled if they fail him.
Split into simpler sentences: After telling Ethan that he has failed to convince Angela to go out with him, Ethan reveals to Dave that he had been obsessing over Angela for quite some time. He reminds Dave that he still intends to get Dave and his friends expelled if they fail him.