He returned to university to continue his studies after completing his military service.

military 是形容词，意思是"军事的、军队的"，来自名词 militia（民兵组织）。 service 是名词，意思是"服务、服役"，来自动词 serve（服务）。 military service 指"兵役"，即公民在军队中服役的义务或经历。