In Quenya a final -ai became -e. The most notable result of this sound change can be seen in the plurals of adjectives end in -a, as indicated in a draft of a 1955 letter to David Masson:

final -ai in plural of adjectives > e (PE17/76).

These notes indicates this sound shift occurred in or shortly before the Tarquesta [TQ] period. The Plotz Letter also mentions this as a late phonetic development: “the diphthong ai was reduced to ǣ > ē > e” (VT6/14), here referring to sound changes for the PQ accusative plural ciryai. This development can be seen in other words that are not adjectives:

• ✶Ossai > Q. Osse (WJ/400).
• ✶waiwa(y) > Q. vaiwe “wind” (PE17/189).
• ✶vai > Q. ve “like, as” (VT49/32).

There are, however, a fair number of Quenya words ending in ai, many of them monosyllables: Q. ai “alas” (LotR/377), hrai “awkward, difficult” (PE17/185), mai “well” (VT47/6). Other are compounds: Q. nai “maybe, be it that” = ná + i (RGEO/60), epetai “consequently” = epe + ta + i (VT49/8), cenai “if it be that” = cé + ná + i (VT49/19). Thus, it seems likely this sound change only occurred when the final -ai was unstressed, as it would be in final syllables of non-compounds and possibly the unstressed preposition ve (though this word had other derivations).