Not a Chance

I was lying in bed early this morning on the off chance that I might be able to fall asleep again (after I had gotten up to take a whiz): this hardly ever happens (the falling-asleep-again, not the taking-a-whiz-at-2-a.m., which happens all the time), but one can hope. Naturally enough, I began to wonder exactly why it might have been that so many negated words begin with "in-" (or "im-", for the labial consonants, or "irr-") and so many other with "un-" and why there doesn't seem to be any rhyme or reason to it, yet another cruel trap for people learning the language. I couldn't figure this out, for the simple reason that nobody can, but I could, and did, mentally make an alphabetical list of some words that begin with the one prefix or the other. (I think I do such things because I can't sleep: Jim thinks I can't sleep because I do such things. Six of one....) After a while I realized I wasn't going to get back to sleep, so as usual I decided to get up and write it all down:

Inanimate but unannounced
Imbalanced but unbaptized
Incredulous but uncrated
Indecipherable but undetectable
Inedible but uneducated
Infallible but unfathomable
Inhospitable but unholy
Inimitable but unimportant
Injudicious but unjust
Immobile but unmoving
Innocent but unnoticed
Inoffensive but unofficial
Impossible but unpopular
Inquiet but unquestioned
Irreversible but unrewarding
Insupportable but unsuspected
Intractable but untraceable

and then I lost interest because I realized I was going to have no luck with "inw-" and "unx-".

There are no negated words beginning with "ing-", "ink-" or "inl-". All "ing-" and "inl-" words use the prefix "in-" as "in", not "not", and all "ink-" words are derived from "ink". Anything after "int-", well, you can look it up yourself if you want to.

Yeah, I'm cheating on a couple of them--"unbalanced" also exists, as does "unquiet"--but the point remains. "Immobile" is right while "unmobile" is wrong, "unmoving" is right while "immoving" is wrong. The only way to make an intelligent guess is to notice that Latin/Romance words, particularly those ending in "-able"/"ible", are more likely to begin with "in-" and Germanic words with "un-", but there are lots of exceptions (such as "untraceable" and "unmanageable"), so mostly you have to learn each form individually, and there are thousands of them. It's like a mean trick we played on anyone learning the language, but it's not intentional: we just seem to have shoved the prefixes on and then let them duke it out to see which one survived the experience.

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By the way, one of the words that got edited out of the list, "impregnable", and I'm sure you have wondered about this before, has absolutely nothing to do with "impregnate". Completely unrelated etymologically. Isn't that a hell of a thing!

"Pregnant" and its offshoots come from "pre-", "before", and the verb "gnasci", "to be born", itself a member of the enormously fecund Indo-European "gen(h)-" family, so "impregnate" is a compounding of four parts: "im-", "in, into", plus "pre" plus "gnasci" plus "-ate", "to do, to make, to act upon".

"Impregnable", on the other hand, consists of "im-", "not", plus the French verb "prendre", to take", in the form of the Old French adjective "prenable", "able to be taken", from Latin "prehendere", "to take, to grasp" (which is also the source of "prehensile", as in the tail of certain primates). The "-g-" doesn't really belong there at all; it was added in the sixteenth century by busybodies who were trying to restore English to the grandeur of Latin, which had given English, through French, such words as "reign" from "regnare", "feign" from "fingere", and "deign" from "dignari". Said busybodies began shoving "-g-" into places it had no business being, such as "impregnable" and "foreign" (from Late Latin "foranus", "outside, outdoors", and therefore unrelated to "reign").