Avestan Old Iranian & Rig Vedic Sanskrit Similarities
The oldest language or dialect in the Avesta, the language of the Gathas and the Yasna Haptanghaiti, is close of the old Sanskrit language used in the Rig Veda, the older Hindu scriptures. Together, they form some of the oldest surviving literature in the world.
The following is an example of the closeness of the Avestan and Sanskrit languages:
Old Iranian/Avestan: aevo pantao yo ashahe, vispe anyaesham apantam (Yasna 72.11)
Old Indian/Sanskrit: abade pantha he ashae, visha anyaesham apantham
Translation: the one path is that of Asha, all others are not-paths.
[The Sanskrit translation of the Avestan was provided to this writer by Dr. Satyan Banerjee.]
Proto Indo-Iranian
The comparative example of the Avestan and Sanskrit languages above demonstrate that the languages are so close that they are for all practical purposes dialects of the same language.
Zoroastrian Avestan Languages