Complex sentence: Honshū's Greater Tokyo Area, which includes the "de facto" capital city of Tokyo and several surrounding prefectures, is the largest metropolitan area in the world, with over 30 million residents.
Split into simpler sentences: Honshū's Greater Tokyo Area, includes the "de facto" capital city of Tokyo and several surrounding prefectures. It is the largest metropolitan area in the world, with more than 30 million residents.