Complex sentence: He was a co-founder of the "Independent on Sunday" in 1989 and edited the paper from 1991 to 1995, when he joined "Granta" as its editor, staying in that post until May 2007.
Split into simpler sentences: He was a co-founder of the "Independent on Sunday" in 1989 and edited the paper from 1991 to 1995. Jack left the title to join "Granta" as its editor, staying in the post until May 2007.