Sindarin Phonetic Development (Part 79)

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S. final [mɸ], [nθ], [ŋx], [lθ] became [mp], [nt], [ŋk], [lt]; [-{mɸ|nθ|ŋx|lθ}] > [-{mp|nt|ŋk|lt}]

In Sindarin and Noldorin, where combinations of nasals and voiceless spirants (mph, nth, nch) became final again after short final vowels vanished, the spirant became a stop again (mp, nt, nc). The same is true of the combination lth (becoming lt) but not rth. Tolkien described this change in a couple places:

Sindarin Phonetic Development (Part 76)

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S. [œi] became [ui] or [y]; [œi] > [ui|y]

One open question is how the diphthong [œi] developed in Sindarin, assuming it occurred at all. This diphthong is relatively common in the Noldorin of the 1930s (where it became [ei]), arising in the phonetic history of plural forms of words with a base vowel o, where the raising of [o] to [u] was usually inhibited in the final syllable of polysyllables:

Warning

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I have been thinking about the verb "to warn". I feel that the attested or- (aorist ora "it warns") is too strongly connected to the idea of inner conscience, metaphorical heart, inner counsel to be used as an active verb outside of its usual impersonal conjugation - so sentences like Túro warned Marko that the water was deep are still not covered.

Sindarin Phonetic Development (Part 74)

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S. [ai], [oi] became [ae], [oe]; [ai|oi] > [ae|oe]

In Sindarin the diphthongs [ai] and [oi] developed into [ae] and [oe] respectively, a set of sound changes noted by David Salo (GS/§4.190). The development [ai] > [ae] applied to both original primitive diphthongs as well as any diphthongs that arose later from the vocalizations of spirants: