Complex sentence: A Boston Landmark is a designation by the Boston Landmarks Commission for historic buildings and sites based on the grounds that it has historical, social, cultural, architectural or aesthetic significance to New England or the United States.
Split into simpler sentences: A Boston Landmark is a designation by the Boston Landmarks Commission for historic buildings and sites. A designated site is selected on the grounds that it has historical, social, cultural, architectural or aesthetic significance to Boston, New England, or the United States.