Complex sentence: They finished in 4th place again for the following 1985-86 season too before finally succeeding in the 1986-87 season, finishing in second place behind Derby County F.C., gaining promotion to Division One for the first time since the 1958-59 season.
Split into simpler sentences: They finished in 4th place again for the following 1985-86 season too. Portsmouth finished the 1986-87 season in second place behind Derby County F.C., gaining promotion to Division One for the first time since the 1958-59 season.