The root √IS was the basis for words having to do with “knowledge” for all of Tolkien’s life, as represented by the verb Q. ista- “to know” which likewise retained the same form and meaning for decades. The root first appeared as ᴱ√ISI in the Qenya Lexicon of the 1910s, where somewhat cryptically Tolkien said its Gnomish form was GIS or IS (QL/43). This is mysterious because there were no such Gnomish words beginning with gis- in the contemporaneous Gnomish Lexicon, but there is an Early Noldorin word ᴱN. gist- “to know” from the 1920s, probably derived from ᴱ✶ʒist- (PE13/144, 146); in this early period initial ʒ- > g- in Gnomish (PE12/17).
Tolkien seems to have abandoned this Noldorin variant, giving the root only as ᴹ√IS in The Etymologies of the 1930s (Ety/IS). In this form it continued to appear in Tolkien’s later writings (PE17/155; PE22/129; VT41/6; VT48/25). In one place Tolkien gave the root in inverted form √SI (PE22/134), and such an inversion appeared in some of its derivatives, such as Q. síma “imagination, mind” (VT49/16) and sinte the irregular past tense of Q. ista-. However, the vast majority of its derivatives are from √IS.
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).