Dan. (unstressed or final?) [ĕ] became [i]; [-ĕ] > [-i]
In some early notes associated with the Comparative Tables, Tolkien indicated that “in East Danian ... e became normally i” (PE19/26). In these notes, Tolkien used the descriptor “West Danian” for phonetic rules ascribed to East Danian in the Comparative Tables (PE19/26 note #85), which might imply that this vowel change applied to West Danian or Ossiriandic. However, there are a number preserved primitive [e] vowels in The Etymologies, such as Dan. Edel < ᴹ✶edel- (Ety/ELED), making it unlikely this rule was a general vowel change.
In the real-world analog of Danian, Proto-Germanic unstressed [ĕ] became [i], so perhaps Tolkien imagined a similar change to unstressed or final short [ĕ] in Primitive Danian. There is some evidence that this is the case. The Danian word for “blue”, lygn < ᴹ✶lugni (Ety/LUG²), shows signs of i-mutation, something that it is not true of any of its cognates in other Eldarin languages. It is known that short final [i] became [e] in Common Eldarin, so that ᴹ✶lugni became ᴹ✶lugne before the divergence of these languages, making it difficult to explain this Danian i-mutation (as noted by Helge Fauskanger, AL-Nandorin/lygn).
If, however, this short [ĕ] reverted back to [i] (or perhaps never changed in the first place, because Danian diverged before the Common Eldarin changes), this would explain the i-mutation in Dan. lygn. Given the lack of examples, however, these developments remain quite speculative. It is also possible that Tolkien’s statements above regarding West Danian were reapplied to Taliska, since that language was said to be of “Gothic” type (PE19/22) and short [ĕ] and [ĭ] merged in that real-world language.
Reference ✧ PE19/26
Phonetic Rule Elements
Phonetic Rule Examples
|lugne > lugni||-ĕ > -i||ᴹ✶lugni > Dan. lygn||✧ Ety/LUG²|