Complex sentence: In 1998, Yamaguchi formed a band with his high school friends from Sapporo Daiichi High School, called Dutchman, which performed British rock-inspired music.
Split into simpler sentences: In 1998, Yamaguchi formed a band with his high school friends from Sapporo Daiichi High School. The band was called Dutchman, and performed British rock-inspired music.