Complex sentence: Believing that Harry needs to learn Voldemort's past to gain advantage in a foretold battle, Dumbledore schedules regular meetings with Harry, in which they use Dumbledore's Pensieve to look at memories of those who have had direct contact with Voldemort.
Split into simpler sentences: Believing that Harry needs to learn Voldemort's past to gain advantage in a foretold battle, Dumbledore schedules regular meetings with Harry. During this time, they use Dumbledore's Pensieve to look at memories of those who have had direct contact with Voldemort.