Why is the adjective “unable” while the noun is “inability”? Why don’t they share the same nucleus/vowel?
WRS
Like so many things in natural languages, that’s just the way it is and there is probably no rational explanation. Possibly, they came from different dialects (which is why the “gh” sound or non-sound has so many different pronunciations). Why “finite”, but “infinite”, something that foreigners seem to have a great deal of trouble learning?
No. The “gh” spelling has so many different pronunciation because people stopped using the sound it represents (the velar fricative – the “ch” in the German “ach” or Hebrew "Chanukah).
An excerpt from un- versus in- by Michael Quinion.
-fh