What linguistic process is illustrated by changing /a/ into /e/ in inars/iners? Assimilation? Why has it taken place?
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2Possible duplicate of If arm is 'arma', why is unarmed 'inermis' and not 'inarmis'? luchonacho’s answer has a quote that mentions iners. – Asteroides Oct 16 '18 at 19:21
This is called vowel reduction. Basically, a vowel that loses emphasis becomes weaker. This is very typical with one-syllable prefixes: ars > iners, facere > efficere. It can also happen due to inflection, as in tango > tetigi (from stem tag- with nasal augment in present stem and reduplication in perfect stem).
Old Latin had initial stress and therefore prefixes move stress away. When stress is lost, a short vowel tends to become weaker. The stress system changed later, but the vowel changes stuck.
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I've always thought of it as vowel raising: early Latin raised most unstressed vowels (a -> e or i, e -> i, o -> u) – Colin Fine Oct 16 '18 at 23:13