Complex sentence: After he sends a servant to spy on the Duc de Nemours, Monsieur de Clèves believes that his wife has been unfaithful in more than just her emotions, and he becomes ill and eventually dies (either of his illness or of a broken heart).
Split into simpler sentences: After he sends a servant to spy on the Duc de Nemours, Monsieur de Clèves believes that his wife has been unfaithful in more than just her emotions. He becomes ill and dies (either of his illness or of a broken heart).