The Indo-Aryan languages have acquired some vocabulary and other features that aren't present in the Iranian languages.
This is ascribed to substrate influence. What is the current understanding of those substrates?
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It is generally accepted that there is a significant Dravidian substrate in Indo-Aryan, and a smaller number of substrate words from Austroasiatic languages like Munda and Khasi. There is a chapter on “Loanwords in Sanskrit” in Burrow’s famous book “The Sanskrit Language”, which you should be able to find in a good library.