This root was the basis for the Quenya future tense suffix Q. -uva. The earliest appearance of this root was as ᴹ√UB “ponder, have in mind” in notes from the early 1940s where it served as the basis for both the future and (archaic) past future suffixes ᴹQ. -uva and ᴹQ. -umne, the future sense derived in Quenya from the meaning “have intention (to do something now or eventually)” (VT48/32). It was also the basis of a noun form ᴹQ. úvie “considering a matter (with a view to decision)”, but according to Tolkien no independent verb form survived in Quenya, though Old Noldorin had ON. ūba- “to brood on, ponder”.

The root ᴹ√UB was not the only proposed explanation for the Quenya future in this period: in Quendian & Common Eldarin Verbal Structure (EVS1) from the late 1940s he gave ᴹ√BĀ/BANA “go, proceed” as the basis for the future suffix ᴹQ. -va (PE22/97), and a similar scenario appeared in the Quenya Verbal System from this same period (PE22/112). In these notes, -va was the basic future suffix, with -uva a vocalic elaboration when used with basic verbs as in kar-u-va “will make, (originally) going to make”.

Tolkien firmly rejected √BA(N) “go” in 1959, however (PE17/149). The root √UB “ponder, have in mind” was again given as the basis for the future suffix in Common Eldarin: Verb Structure (EVS2) in the early 1950s, with a scenario similar to the 1940s note mentioned above, but with a slightly different archaic future-past construction: AQ. †umbeste mate “he was going to eat” (PE22/132). The root √UB “consider, have in mind” was mentioned in passing in notes from 1968 (VT48/25). In Late Notes on Verb Structure (LVS) from 1969, Tolkien again described the origin of the Quenya future:

Q. developed a pure future of fact or eventuality, with a stem ubā suffixed to the bare base (without ómataima): as *karubā- “will do, is going to do”. This base UB provided a verbal stem (an a-verb) uba- in original sense: impend, be imminent, approach, draw near. In this form it was at an early date affixed to the simple verbal stem, but with a weakened and generalized sense: as e.g. *kar|ubā|ni = “I draw near to doing, I am about to do (it)” > CQ [Classical Quenya] caruvan(ye) “I am going to do/shall do (it)” (PE22/167).

The independent (and impersonal) verb Q. ūva “impend, be imminent” took on the sense “threaten (to come)” due to the “bad” sense of the ū-prefix, and likewise for the noun form Q. umbë “imminence, threat” (PE22/167, 168). In this note Tolkien said derivatives of this root did not survive at all in Sindarin.

Neo-Eldarin: For purposes of Neo-Eldarin, I would retain the basic sense “ponder, have in mind” for the root √UB that Tolkien used from the 1940s up to 1968, and would assume the 1969 meaning “impend, be imminent” was an extension of that sense, as a thing being pondered. As such, I’d allow the “ponder” meaning to survive in Sindarin.

See √UB for discussion.