Complex sentence: Quite early in the pre-historic era, Sanskrit separated from the Avestan language (an Eastern Iranian language), but the exact century of separation is unknown but this occurred certainly before 1800 BCE.
Split into simpler sentences: Quite early in the pre-historic era, Sanskrit separated from the Avestan language (an Eastern Iranian language). The exact century of separation is unknown but this separation of Sanskrit and Avestan occurred certainly before 1800 BCE.