How are Iranian languages related to Indo-Aryan languages?

Iranian and Indo-Aryan languages are closely related, forming the Indo-Iranian subgroup of the Indo-European language family. Their shared ancestry is evident in significant linguistic and cultural commonalities.12

Common Features

The two groups share distinct phonological and grammatical traits, such as the development of Proto-Indo-European vowels and specific religious vocabulary, including terms for ritual offerings and shared mythological figures like Mithra. Speakers of both branches historically used similar terms to refer to themselves as Aryan.1

Divergent Features

Despite these similarities, the branches have evolved distinct differences:3
* Phonology: Iranian languages replaced Indo-Iranian aspirated voiced consonants with unaspirated ones and often replaced the sound s with h. Additionally, Iranian languages were more conservative in retaining early diphthongs like ai and au, which shifted to simple vowels in Indo-Aryan.3
* Grammar: The two groups exhibit differences in pronoun usage and case endings, such as the dative singular of -a- stems.3

These languages have played foundational roles in major world religions, with Indo-Aryan serving as a vehicle for Hinduism and Buddhism, while Iranian languages are central to Zoroastrianism.4