√DAT root. “fall down, fall to ground”
The Quenya word for “fall” was Q. lanta- for all of Tolkien’s life, and in his later writings it was derived from the root √DAT, where the initial [d] became [l] as it usually did in Quenya. But this was not the earliest root for “fall”, which was given as ᴱ√LANTAN in the Qenya Lexicon of the 1910s (QL/51), though the actual primitive form was probably *ᴱ√LṆTṆ. This early root is consistent with the Gnomish form G. lantha- “fall onto, settle on, alight” (GL/52). Indeed, this is the most reasonable root in this early period, since initial [d] > [t] in Early Qenya rather than to [l] as it did later on.
In The Etymologies of the 1930s, however, Tolkien gave the root as ᴹ√DAT “fall down” (Ety/DAT), replacing a deleted root ᴹ√LANTA (Ety/TALÁT; EtyAC/LANTA). Tolkien coined a new Noldorin verb for “to fall”, as N. dant- with passive participle N. dannen “fallen”. The Noldorin verb form dant- was probably archaic or incomplete, and with the proper form most likely matching its later Sindarin form, S. danna- (PE17/62).
The root √DAT “fall” appeared pretty regularly in later notes (VT47/29; VT48/24, 30). In some notes associated with words in The Lord of the Rings, Tolkien gave a new derivation of Q. lanta- and S. danna- from a root DAN-TA (PE17/62), but I suspect this was just the nasal-infixed form of √DAT, as with the identical form ᴹ√DANT from the 1930s (Ety/DAT).
However, in notes from 1968, Tolkien gave √LAT as a variant of √DAT via ancient d/l interchange:
d and l interchanged frequently in early Common Eldarin, as exemplified by Dat/Lat “fall (to the ground)” (VT47/29).
Detecting this variant in Quenya would be nearly impossible, but it is probably evident in the Sindarin word for “waterfall”: S. lanthir (S/235; PM/349), perhaps < *lat-sirē “fall-river”. This variant might be a partial restoration of the early root ᴱ√LṆTṆ, but there no clear signs of it outside this 1968 note and the Sindarin word for “waterfall”.
References ✧ PE17/62, 151; VT47/29; VT48/24, 30
ᴹ√DAT root. “fall down”
References ✧ Ety/DAT, PEN, TALÁT; EtyAC/DAT, LANTA
*ᴱ√LṆTṆ root. “*fall”
Reference ✧ QL/51 ✧ LANTAN