Complex sentence: The Air Force had also been studying a lighter day fighter; starting in 1965, the Air Force had pursued a low-priority study of the Advanced Day Fighter (ADF), a 25,000 pound design.
Split into simpler sentences: The Air Force had also been studying a lighter day fighter. Starting in 1965, the Air Force had pursued a low-priority study of the Advanced Day Fighter (ADF), a 25,000 pound design.