A commandant is the army officer in charge of a place — a camp, a fort, a prison, a military school — or of the people who live and work there. It is more a job title than a everyday rank: you are the commandant of somewhere. Commander is the more common word for someone who leads soldiers in the field; commandant sounds a bit more formal and is usually tied to a place. Though actually, in some armies the same word is also a rank (roughly like a major), but in ordinary English you mostly meet it as “the officer who runs this place.”

指揮官；司令官（多指駐地、營區、軍校的主管，不是戰場上帶兵的一般 commander）