Complex sentence: After graduation he volunteered for military service and completed his training in Armoured Warfare Training Centre (Centrum Wyszkolenia Broni Pancernych) in Modlin in 1937, then he began to study at the Warsaw Polytechnic.
Split into simpler sentences: After graduation he volunteered for military service and completed his training in Armoured Warfare Training Centre (Centrum Wyszkolenia Broni Pancernych) in Modlin in 1937. Then he began to study at the Warsaw Polytechnic.