Complex sentence: He was commissioned, and seconded into the RFC in September 1917 and posted back to England for flight training, serving in England as a flying instructor until August 1918 when he transferred to the Western Front.
Split into simpler sentences: He was commissioned, and seconded into the Royal Flying Corp (RFC) in September 1917. Posted back to England for flight training, he served there as a flying instructor until August 1918, when he transferred to the Western Front.