This root is the basis for Elvish question words. When it first appeared in the Gnomish Lexicon of the 1910s, Tolkien said ᴱ√MA was the “root of indef[inites]” (GL/55), and it served as the basis for the indefinitive suffix G. -(u)m and ᴱQ. -(u)ma (GL/55; PE14/42, 71). In the Gnomish Lexicon, question words were derived from the root ᴱ√DO instead: G. don “who”, G. dos “when”, G. dui “where”, etc. (GL/30). There are no signs of interrogative ᴱ√DO in Tolkien’s later writings, however.
Indeed, by the time Tolkien wrote the ᴱQ. Oilima Markirya poem around 1930, Quenya question words generally began with ma-: ᴱQ. man kiluva kirya ninqe? “Who shall see a white ship?” (MC/213). This remained true thereafter, and the same seems to be true of Sindarin as well, though we have far fewer examples: S. man agorech?, untranslated but apparently: “✱and said Rían to Tuor: what have we done?” (VT50/5). The “interrogative base” √MA is mentioned in Quenya Notes from 1957 (QN: PE17/162) and again in notes from around 1968 (VT47/19).
√MA does not seem to be completely divorced from indefinites in Tolkien’s later writings, however, given his use of Q. ma for an indefinite neuter pronoun “(some)thing” as opposed to the indefinite personal pronoun Q. mo “(some)one” (PE22/154; VT42/34). Whether these indefinites are based on the same root or a similar but unrelated root isn’t clear.