Complex sentence: Since 1991, in all parliamentary elections, he had been being nominated as the candidate of BNP from Khagrachari constituency except 10th because of his imprisonment during military influenced caretaker government's period in 2007--2008.
Split into simpler sentences: Since 1991, in all parliamentary elections but 10th, he had been nominated as the candidate of BNP from Khagrachari constituency. In 10th parliamentary election, he couldn't participate due to his imprisonment during military influenced then-caretaker government's period in 2007--2008.