The Quenya verb for “to fall”, dating all the way back to the Qenya Lexicon of the 1910s, though in that document it had the form ᴱQ. lant- and the additional gloss “drop” (QL/51). In the Qenya Lexicon it was derived from the early root ᴱ√LANTAN [LṆTṆ], but in The Etymologies of the 1930s Tolkien changed the root {ᴹ√LANTA >>} ᴹ√DA(N)T “fall down” as the basis for ᴹQ. lanta- “to fall” (Ety/DAT; EtyAC/LANTA). Q. lanta- “fall” appeared regularly in Tolkien’s later writings as well (MC/222; PE17/62; VT49/47), most notably in the Namárië poem in its first phrase: ai! laurië lantar lassi súrinen “ah! like gold fall the leaves in the wind” (LotR/377).
The word lanta was occasionally used as a noun for “a fall”; see that entry for discussion.
Neo-Quenya: In Tolkien’s later writings, lanta- was used only for intransitive “fall”. However, we have no Quenya verbs for transitive “drop” other than 1910s ᴱQ. lant-, so I would assume that lanta- can be used this way as well for purposes of Neo-Quenya (QL/51). For example, lantan “I fall” vs. lantan i macil “I drop [make fall] the sword”.
A Sindarin verb for “fall” in Notes on Galadriel’s Song (NGS) from the late 1950s or early 1960s, cognate to Q. lanta- and derived from √DAN-TA (PE17/62). Elsewhere the root for “fall down” was √DAT (VT47/29; VT48/24; Ety/DAT), so √DAN-TA was probably a nasal-infixed variant of the root; compare ᴹ√DAT vs. ᴹ√DANT from The Etymologies of the 1930s (Ety/DAT). In Sindarin, medial ancient nt became nn, so ✱danta- > S. danna-.
Conceptual Development: The Gnomish Lexicon of the 1910s had G. {lanta- >>} lantha- “fall onto, settle on, alight” (GL/52). It had a past form lantathi with a light pencil stroke through it indicating was thus tentatively rejected. This Gnomish verb is clearly cognate to ᴱQ. lant- “drop, fall” from the contemporaneous Qenya Lexicon of the 1910s under the early root ᴱ√LANTAN [LṆTṆ] (QL/51).
The Etymologies of the 1930s had the root {ᴹ√LANT >>} ᴹ√DAT or ᴹ√DANT “fall down” (Ety/DAT, TALÁT; EtyAC/LANTA). Under the entry for ᴹ√DAT, Tolkien had N. dant- “to fall” with passive participle dannen “fallen” (Ety/DAT). Likely N. dant- was a stem form which would become dann- when inflected, since in Noldorin of the 1930s and 40s ancient medial nt also became nn, as it did in Sindarin.