Complex sentence: Year 300 BC was a year of the pre-Julian Roman calendar.In the Roman Empire it was known as the Year of the Consulship of Corvus and Pansa (or, less frequently, year 454 "Ab urbe condita").
Split into simpler sentences: Year 300 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Corvus and Pansa (or, less frequently, year 454 "Ab urbe condita").