Qenya
naina-
verb. to lament, to lament, *mourn
(nai)naina-
verb. to lament
nui
noun. lament
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A verb in The Etymologies of the 1930s appearing as (nai)naina- “lament” derived from the root ᴹ√NAY of the same meaning (Ety/NAY). In The Etymologies as published in The Lost Road Christopher Tolkien gave the form as naina- (LR/375), but Carl Hostetter and Patrick Wynne corrected this to (nai)naina- in their Addenda and Corrigenda to the Etymologies (VT45/37). The ongoing validity of this verb is indicated by the noun form nainië “lament” in the title of Galadriel’s poem: Altariello nainië Lóriendesse (RGEO/58).
Conceptual Development: A likely precursor is ᴱQ. nyéna- “lament” in the Qenya Lexicon of the 1910s under the early root ᴱ√NYE(NE) “bleat” (QL/68).
Neo-Quenya: In a 2024-11-03 post on the Vinyë Lambengolmor Discord Server (VLDS), Luinyelle suggested this verb might also mean “✱mourn”.