Bare 未 negates a single not-yet-done event (未完成, still pending). 未曾 adds 曾 to negate the whole prior span — not once across all of it.
不曾
未曾 and 不曾 are near-twins for 'has never'. 不曾 stresses the plain fact it did not occur; 未曾 carries the 'up to now, still not' flavor of 未.
✗ 我未曾去明天。 → ✓ 我未曾去過那裡。 (未曾 looks back over past experience; it cannot point at the future)
✗ 他未曾不來。 → ✓ 他未曾來過。 (未曾 already carries the negation; don't stack a second 不)
✗ 這件事未曾。 → ✓ 這件事未曾發生過。 (未曾 needs the verb it negates right after it; it cannot stand alone)
English 'never' sits as one adverb anywhere; learners drop 過 or place 未曾 loosely. 未曾 must sit immediately before the verb, and the experiential 過 usually follows.