Complex sentence: In December 1939, as one of the first Polish fighter pilots to arrive in Great Britain, he underwent training on the Supermarine Spitfire and was commissioned as Flying Officer (F/O) and posted to No.65 "East India" Squadron at RAF Hornchurch.
Split into simpler sentences: In December 1939 he was one of the first Polish fighter pilots to arrive in Great Britain. He underwent training on the Supermarine Spitfire and was commissioned as Flying Officer (F/O) and posted to No.65 "East India" Squadron based at RAF Hornchurch.