Complex sentence: Iolani Palace, in the capitol district of downtown Honolulu in the U.S. state of Hawaii, is the only royal palace in the United States used as an official residence by a reigning monarch and is a National Historic Landmark listed on the National Register of Historic Places.
Split into simpler sentences: Iolani Palace, in the capitol district of downtown Honolulu in the U.S. state of Hawaii, is the only royal palace now a part of the United States. It was used as an official residence of the reigning Hawaiian sovereign and is now a National Historic Landmark listed on the National Register of Historic Places.