Complex sentence: Carboy unfastens his jacket, and the crowd are shocked to find he is a Knight of the Garter who had disguised himself as a commoner so that he could woo Eliza.
Split into simpler sentences: Carboy unfastens his jacket, and the crowd are shocked to find he is a Knight of the Garter. He had disguised himself as a commoner so that he could woo Eliza.