√THIL “shine silver”
Tolkien frequently used the root √THIL as the basis for “moon” words in Elvish. The first appearance of this root was ᴱ√ÞILI a variant of ᴱ√SILI “*gleam” from the Qenya Lexicon of the 1910s (QL/83). The normal “moon” words in this period were ᴱQ. Sil and G. Sil (QL83; GL/67). A rejected Gnomish variant Thil “the moon” indicates Tolkien considered ᴱ√ÞILI, but did not commit to it (GL/72). However, in The Etymologies of the 1930s Tolkien gave unglossed ᴹ√THIL with derivatives like ᴹQ. Isil and N. Ithil “Moon, (lit.) Sheen” (Ety/I²; THIL). Tolkien stuck with these “Moon” words thereafter, and the root √THIL was mentioned regularly in Tolkien’s later writings (Let/425; PE17/66; PE22/136).
√THIN “*grey”
Tolkien introduced the (unglossed) root ᴹ√THIN in The Etymologies of the 1930s with derivatives like ᴹQ. sinde/N. thinn “grey, pale” and ᴹQ. sinta-/N. †thinna- “fade” (Ety/THIN). In this document it was the basis for the name Ilk. Thingol; in earlier writings from the 1920s the name ᴱN. Thingol was based on the word ᴱN. thing “prince” (PE13/154). The root √THIN or √ΘIN was mentioned a couple times in Tolkien’s later writings as the basis for “grey” words (PE17/72; WJ/384). In a note from the mid-1960s Tolkien considered making the root √STIN the basis for “grey”, as a privative formation = √S-TIN = “without sparkling” (PE17/184), but I think this was a transient idea.
√THOL “stand up, top; helmet”
In Quenya Notes (QN) from 1957 Tolkien gave the root √STOL “helmet” with derivatives like S. thôl or Q. castol of the same meaning (PE17/186). In etymological notes from around 1964 (DD) Tolkien instead gave √ÞOL “stand up, top” as the basis for these “helmet” words (PE17/188).
ᴹ√THON “*pine”
The unglossed root ᴹ√THON appeared in The Etymologies of the 1930s with derivatives Ilk. thōn and N. thaun “pine-tree” (Ety/THOL); according to Hostetter and Wynne, Christopher Tolkien incorrectly recorded this root as ᴹ√THŌN, but that was actually just the Ilkorin form of the word. Tolkien explained the somewhat unusual Noldorin form N. thaun as the result of a loan for Ilk thōn, with the Noldorin word being adapted as ō [ǭ] < [primitive] ā; this loan must have occurred in the period after ancient long [ō] became [ū] in Old Noldorin, but before [ǭ] (from [ā]) became [au]. This 1930s root was used mainly to explain the name Ilk. Dorthonion “Land of Pines”.
In a 1955 letter to David Masson, Tolkien derived S. thôn “pine” from ✶stŏna (PE17/82). However, in Words, Phrases and Passages from the Lord of the Rings written in the late 1950s or early 1960s Tolkien derived this element of S. Dorthonion from {stāna >> thānĭ >>} thŏno, apparently coming full circle back to the 1930s root √THON (PE17/81).
ᴹ√THOR(ON) “come swooping down”
Tolkien used similar Elvish words for “eagle” for much of this life. The first appearance of its root was ᴱ√ŠORO⁽²⁾ “eagle” from the Qenya Lexicon of the 1910s with derivatives like ᴱQ. sorne “eagle” and ᴱQ. sornion “eyrie” (QL/86). Gilson, Hostetter, Wynne and Smith suggested that the Š may represent more ancient Þ, and Gnomish derivatives from the contemporaneous Gnomish Lexicon such as G. thorn “eagle” and G. thrond “eyrie, pinnacle” confirm that the actual early root was *ᴱ√ÞORO (GL/73). In The Etymologies of the 1930s Tolkien gave the root as ᴹ√THOR or ᴹ√THÓRON “come swooping down” with derivatives like ᴹQ. soron/N. thoron “eagle” and N. thôr “swooping, leaping down” (Ety/THOR). In a 1972 letter to Richard Jeffery, Tolkien gave primitive form ✶thorono “eagle” as the basis for Q. sorno/S. thoron, testifying to the enduring nature of this root (Let/427).
√THOS “frighten, terrify; show dread of”
In etymological notes from the late 1950s or early 1960s, Tolkien gave √ÞOS “frighten, terrify” as the basis for the final element of S. guruthos, along with other derivatives like OQ. þossë/S. thoss “fear” and OQ. þosta- “put to fright, terrify” (PE17/87). In other notes from this period Tolkien gave √ÞOS “show dread of”, again as the basis for the final element of S. guruthos “shadow of death” (PE17/95). In The Etymologies of the 1930s, the form N. guruthos “fear of death” appeared a marginal notes to the entry for ᴹ√GOS “dread”, indicating the final element of that word was original derived from ᴹ√GOS (EtyAC/GOS); in this instance its initial element “death” was likely N. guruth rather than S. guru.
√THŪ “[ᴹ√] puff, blow”; √SŪ “wind”
Tolkien used a variety of related roots for “wind” and “breath”, intermingled with the name Q. Súlimo, surname of Manwë. The earliest of these roots was ᴱ√SUHYU “air, breath, exhale, puff” with variants ᴱ√SUHU and ᴱ√SUFU from the Qenya Lexicon of the 1910s, having derivatives like ᴱQ. sú “noise of wind”, ᴱQ. súma “nostril”, and ᴱQ. †súye “air, breezes, winds” (QL/86). Related forms in the contemporaneous Gnomish Lexicon confirm that the root began with S-, such as: G. saul “a great wind”, G. sû, G. and sûtha- “blow (of wind)” (GL/67-68). In Early Noldorin Word-lists from the 1920s Tolkien twice mentioned ᴱ√suru “(to) blow” as the basis for ᴱN. huiriaith “gale” (PE13/148, 163).
In The Etymologies of the 1930s Tolkien instead had ᴹ√THŪ “puff, blow” with derivatives like ᴹQ. súle/N. thûl “breath” and ᴹQ. súya-/N. thuia- “breathe” (Ety/THŪ). A document grouped with Quenya Notes (QN) from 1957 had an extensive discussion of Q. súlë “spirit; breath” where he gave the root as √THŪ with variant √SŪ (PE17/124). The root √SŪ was also mentioned in Notes on Galdriel’s Song (NGS) from the late 1950s or early 1960s, along with extended form √SUR(U) serving as the basis for Q. súrë “wind” (PE17/62). In notes from the late 1960s, ✶sū was mentioned a couple times as an example of an ancient monosyllabic noun with the meaning “(noise of) wind” (VT47/12, 35). In one of these documents Tolkien clarified that base form ✶sū was “used only of the sound of wind; as noun [for actual ‘wind’] + r/l: suli, sur(i)” (VT47/12). Compare Q. súrë above as well as S. sûl “wind” (PE17/15; RC/778).
Thus it seems clear that Tolkien had two distinct but possibly related roots: √THŪ “breath” and √SŪ “(noise of) wind”.
ᴹ√THUR “surround, fence, ward, hedge in, secrete”
A root in The Etymologies of the 1930s glossed “surround, fence, ward, hedge in, secrete”, with derivatives like N. thora- “fence” and Ilk. thúren “guarded, hidden”, the latter serving as an element in the names Ilk. Thuringwethil “(Woman of) Secret Shadow” and Ilk. Garthurian “Hidden Realm” (Ety/THUR). Both these names survived in later versions of The Silmarillion (S/178; WJ/189), and later names like S. Thurin “Secret” and S. Thuringud “Hidden Foe” (UT/157; WJ/256) imply the ongoing validity of this root, though the survival of u in S. thurin is rather mysterious; David Salo suggested the ancient form might originally have been thūrin(e) (GS/291).
The earliest precursor to Garthurian was G. Gar Furion “Secret Place” from the 1910s (PE13/102; PE15/24), which was based on the unglossed root ᴱ√FURU or ᴱ√HURU from the Qenya Lexicon of the 1910s with derivatives like ᴱQ. furin/G. furion “secret, hidden, concealed” and Q. furu/G. fûr “a lie” (QL/39; GL/36). In the Lost Tales of this period, Tolkien changed {G. Gar Furion >>} G. Gar Thurion (LT2/202), so it seems the shift from ᴱ√FURU >> *√ÞURU was very early.