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.
ista- (2) vb. "know", pa.t. sintë (IS, LT2:339, VT48:25). This past tense Tolkien called "certainly irregular" (VT48:25, where an alternative pa.t. isintë is also mentioned, but sintë is said to be the older form; compare editorial notes in VT48:32. Ista- is also used for "can" in the sense of "know how to", as in istan quetë "I can speak (because I have learned (a) language)" (VT41:6) Passive participle sinwa "known, certain, ascertained" (VT49:68)