Complex sentence: The Italian Renaissance had rediscovered the ancient Greek and Roman theatre, and this was instrumental in the development of the new drama, which was then beginning to evolve apart from the old mystery and miracle plays of the Middle Ages.
Split into simpler sentences: The Italian Renaissance had rediscovered the ancient Greek and Roman theatre. This was instrumental in the development of the new drama, which was then beginning to evolve apart from the old mystery and miracle plays of the Middle Ages.