Complex sentence: After their 1994 breakthrough, Green Day's fame was fading, mainly due to rising popularity of other bands like Good Charlotte and Sum 41, realising this they retreated to the studio and produced their seventh studio album "American Idiot" released in 2004.
Split into simpler sentences: After their 1994 breakthrough, Green Day's fame was fading, mainly due to rising popularity of other bands like Good Charlotte and Sum 41. Realising this, they retreated to the studio and produced their seventh studio album "American Idiot" released in 2004.