Complex sentence: Since 1955, when this summer festival first began, the Greek National opera has presented special performances for the ancient Roman-era amphitheatre, the Odeon of Herodes Atticus; its first presentation was Gluck's "Orfeo ed Euridice".
Split into simpler sentences: Since 1955, when this summer festival first began, the Greek National opera has presented special performances for the ancient Roman-era amphitheatre, the Odeon of Herodes Atticus. Its first presentation was Gluck's "Orfeo ed Euridice".