tolkienlanguages1-6  Exercise 1: Plurals
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  adan    -> edain     men      (vowel change inside the word)
  orch    -> yrch      orcs     (vowel change)
  galadh  -> gelaidh   trees    (vowel change)

The change is i-affection: a vowel is altered by an i that used to follow it.
English "man -> men" is the same kind of change, a vowel altered inside the
word, not an ending added to it. Quenya, by contrast, adds an ending
(atan -> atani).