Complex sentence: When they leave for training camp, Jack mistakenly believes Sylvia prefers him; she actually prefers David and lets him know about her feelings, but is too kindhearted to turn down Jack's affection.
Split into simpler sentences: When they leave for training camp, Jack mistakenly believes Sylvia prefers him. She actually prefers David and lets him know about her feelings, but is too kindhearted to turn down Jack's affection.