Complex sentence: Grundel, who is searching for Thumbelina, finds Beetle and discovers Thumbelina is in love with Prince Cornelius and upon some convincing from Beetle, decides to find and kidnap Cornelius to lure Thumbelina to him.
Split into simpler sentences: Grundel, who is searching for Thumbelina, finds Beetle and discovers Thumbelina is in love with Prince Cornelius. Upon some convincing from Beetle, Grundel decides to find and kidnap Cornelius to lure Thumbelina to him.