Complex sentence: When Lucy begins to waste away suspiciously, Seward invites his old teacher, Abraham Van Helsing, who immediately determines the cause of Lucy's condition but refuses to disclose it but diagnoses her with acute blood-loss.
Split into simpler sentences: When Lucy begins to waste away suspiciously, Seward invites his old teacher, Abraham Van Helsing, who immediately determines the true cause of Lucy's condition. He refuses to disclose it but diagnoses her with acute blood-loss.