Tolkien used √SI as the basis for “near demonstratives” like “here” and “now” from very early in his writings on Elvish. The Qenya Lexicon of the 1910s had two competing roots ᴱ√HYA “this by us” with derivatives like ᴱQ. hyá “here by us” (QL/41) and ᴱ√KI “this by me” with derivative ᴱQ. tyá (< ᴱ✶kı̯-ā) “now” (QL/41, 49). Indications of the latter can be seen words in the contemporaneous Gnomish Lexicon such as G. cî {“now” >>} “here” and G. cîrin “present (place or time), modern” [gloss deleted] (GL/26). However, Tolkien also introduced a new root ᴱ√si(n) “this here by me” with derivatives like G. sî “here” and G. sith “hither” (GL/68). Revisions of Gnomish ci- word glosses indicate Tolkien was vacillating on which forms were temporal and which were spatial.
In The Etymologies Tolkien gave the root ᴹ√SI “this, here, now” with derivatives like ᴹQ. sí or sin “now” and ᴹQ. sinya/N. sein “new” (Ety/SI). The root √SI was mentioned a couple times in Tolkien’s later writings, usually glossed “this” (PE17/67; VT48/25; VT49/18) and in one place with the variant √SIN (PE17/67). This root was not entirely without competition in Tolkien’s later notes, however: in one place he gave primitive ✶khĭn- as the possible basis for Q. hí “here” and S. hí “now” in 1968 notes on demonstratives, though it appeared beside primitive ✶si- forms (VT49/34 note #21).
Tolkien used the root √SIL and its variant √THIL for Elvish words for shining things for most of his life. This root first appeared in the Qenya Lexicon as unglossed ᴱ√SILI with variant ᴱ√ÞILI and derivatives like ᴱQ. Sil “Moon”, ᴱQ. sili- “gleam, glint”, ᴱQ. silik “flint”, and ᴱQ. silqe “glossy hair” (QL/83), though last of these was the result of blending with ᴱ√SḶKḶ “rich, lush”, variant of ᴱ√SṚKṚ “fat” (QL/86). The contemporaneous Gnomish Lexicon had derivatives like G. Sil “Moon” and G. silc “flint” pointing only to ᴱ√SILI (GL/67); the only indication of variant ᴱ√ÞILI was in unglossed G. thil followed by deleted forms like G. Thil “Moon” (GL/72).
By The Etymologies of the 1930s, the variants ᴹ√SIL “shine silver” and ᴹ√THIL were better established (Ety/SIL, THIL). The variant ᴹ√SIL had derivatives like ᴹQ. silme/N. silif “moonlight, light of Silpion” and the extended form ᴹ√SÍLIP from which ᴹQ. Silpion was derived (Ety/SIL), and its most notable use from the 1930s forward was as the basis for the initial element of Q. Silmaril vs. S./N. silef as in N. Silevril (Ety/SIL; PE17/23; Let/425). The variant ᴹ√THIL had derivatives like ᴹQ. Isil/N. Ithil “Moon, (lit.) the Sheen” (Ety/THIL), forms Tolkien also retained from the 1930s forward. Tolkien mentioned both root variants √SIL and √THIL with the glosses like “shine (white or silver)” regularly in his later writings (Let/425; PE17/66; PE22/136; SA/sil).