A root implied by various Quenya words having to do with “judgement” from the 1950s and 60s, most notably Q. Námo “Ordainer, Judge” as the true name of Q. Mandos (S/28), a name that began to appear in documents starting in the early 1950s (PE21/85). The root is evident in other words from this period, such as Q. namna “statute” (MR/258), Q. námië “a single judgment or desire” (VT41/13) and the verb Q. nam- “judge” in the phrase: Q. ore nin karitas nō namin alasaila “I feel moved to do so but judge it unwise” (VT41/13). ✱√NAM might also be the basis for the second element of the name Q. Rithil-Anamo “Doom-ring”, the circle of thrones where the Valar sat in council (WJ/401).
In notes from 1969 Tolkien seems to have changed his mind of the root for “judge”, writing Q. nemin >> Q. hamin >> Q. navin for “I judge”, and giving a new root √NDAB “to judge” in a marginal note along with a revised name Návo to replace Námo (PE22/154, notes #53 and #55). This new root conflicts with √NDAB “endeavor, try” from earlier in the same bundle of documents (PE22/151).
Neo-Eldarin: For purposes of Neo-Eldarin, I would ignore the late change of ✱√NAM >> √NDAB “judge”, since ✱√NAM has more attested derivatives and Námo appears in the published Silmarillion. Also, all of the derivatives of this root are Quenya, and its possible use in the name Rithil-Anamo (coined in Valinor) imply that it might be a root invented after the Elves arrived in Aman. Thus I think it is best to treat it as Quenya-only root, and used ᴹ√BAD “judge” as the basis for (Neo) Sindarin words for judgement by retaining Noldorin words with these meanings from the 1930s (Ety/BAD).
This root appeared as ᴹ√NIM “it appears, occurs to (one’s mind)” in the first version of notes on Quenya Personal Pronouns (QPP1) from the late 1940s, with modern Quenya nimin(ye) “it seems to me” (PE23/89). In Quendian & Common Eldarin Verbal Structure (EVS1) from this same period, Tolkien had nemini “appears/seems to me”, revised to nimini, which was deleted and a new paragraph was written with nemin “it seems to me” (PE22/93 note #6 and #7).
In the Quenya Verbal System from 1948 Tolkien used nem- (PE22/99-100, 105, 111), and in Common Eldarin: Verb Structure (EVS2) from the early 1950s had Q. {nemeste} nemesta “appearance, seeming” (PE22/137). Eldarin Pronouns (EP1) also from the early 1950s had the root √NEM as the basis for ✶nemi-ne “it appears to me, me seems” (PE23/123). The forms Q. níma/nimulë and S. nîf/nivol appeared in 1957 notes, all glossed “phantom” or “seeming”, indicate Tolkien may have restored ✱√NIM “seem”. I prefer to stick with better attested √NEM for purposes of Neo-Eldarin.