sindarin_grammar1-4  Exercise 2: Undoing Mutation
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  i chin       -> hin (plural of hen, "child")         h -> ch
  i thiw       -> tiw (plural of tew, "sign")          t -> th
  i melegyrn   -> belegyrn (plural of belegorn)        b -> m
  i mbair      -> bair (plural of bar, "home")         b -> mb

All four follow the plural article in, which lost its n before a consonant.
The vowels of the plural stems had already changed (Chapter 3); mutation
then changed the first letter.