I recently witnessed one of those lightbulb illuminating moments when someone suddenly "got it."  What this language learner "got" was the difference between adjectives and nouns prefixed with un-, and verbs prefixed with un-. The adjective/noun becomes negative, but the verb typically has its action reversed: unusual vs. unwrap, for example.

For most students, learning prefixes doesn't present much of an intellectual challenge, as the concept of adding components to an existing word to modify it or change its meaning is familiar to most languages. The negative prefix un- in English, for example, is shared with Germanic languages. The problem lies not with the idea, but with its implementation, as is so often the case. Which prefix to use in which situation?  Even in its simplest form, the humble prefix can present problems, and forming the negative isn't always as easy as we might imagine. To start with, though un- is the most common negative prefix — undo, unnecessary, unmanageable, unthinkable, unusual — and provides us with 26% of the total, there are others jostling for attention, such as in- (inappropriate),  im- (immobile),  il- (illegal), and ir- (irrelevant). When added to un- these make up nearly half of all negative prefixes.

When we would use ir- seems simple enough: before an r (irresistible, irreducible, irreconcilable), but not to be confused with words like iridescent, which is not a negative and has only one r, or irate which similarly only has one r, but which comes from the word ire and is not some negative connotation of rate. Il- seems happy enough to link up with another l (illegible, illiterate, illegitimate), so we might think that im- is always put before another m, but what of  impolite, impossible and impassable? Does this mean that p is another letter to attract im-?  What about unpopulated then?  Confusing? All this seems to indicate that there are long lists of words to learn, with a few guidelines or rules which are then quickly broken.

As it happens, the digital age has increased the prevalence of  un- as a negative prefix. There was not a little controversy when Facebook introduced the concept of unfriend when some argued that defriend would be more accurate — de- being more frequently used to denote the reversal of something already achieved rather than the simple negative. Unfriend caught on, though, probably because un- was well established in the terminology of reversing computer actions: undo, unerase, undelete, etc. (For more, see Ben Zimmer's New York Times Magazine column, "The Age of Undoing," and his Word Routes followup, "The Un-Believable Un-Verb.")

Incidentally, unfriend appeared as a noun as early as the 13th century, and Shakespeare used unfriended as an adjective in "Twelfth Night," so it already had a venerable history before Facebook made it so universal. Defriend didn't have the same lineage and hasn't become as popular as unfriend, though both are to be found in the social networking world. All this helps to reinforce the tendency to use un- as the default negative prefix. This is further reinforced historically by the imposing figure of Shakespeare, that profligate coiner of language, who used un- to prefix words to reinforce a situation, choosing words that shouldn't really take a negative form for treatment as dramatic effect — My death's sad tale may yet undeaf his ear comes from "Richard II," while elsewhere Lady Macbeth asks the spirits to unsex her, and Malcolm claims that he is going to unspeak what he has said. In Lewis Carroll's Through the Looking Glass, we are told by Humpty Dumpty that "There are three hundred and sixty-four days when you might get un-birthday presents... and only one for birthday presents, you know."

There is the added problem with words that appear to have a prefix but, should the "prefix" be subtracted, then the stem would  appear not to exist at all. Most are, in fact, cases of mistaken identity, where the prefix is not, perhaps, the one that was imagined. In this category we might find universal, unique, and university, where the prefix is not un- but uni-, deriving from the Latin rather than Germanic, and meaning "one."  Unanimous (single spirit or resolution) would also fall into this category. Or else we might be looking at words like undulate, which derives from unda, the Latin for wave.

On the other hand, there are a few examples of words where originally only the negative form existed but for which the positive has been invented (a method that dictionary compilers call back-formation), or where the negative form is fairly common but the positive not so much. Examples of these would be unruly (thus ruly — orderly, well behaved; a nineteenth century invention) and unravel (thus ravel — tangle or jumble; derived from an obsolete Dutch word). There are a few other traps, such as words that are used in more than one context and which have a positive version in one context but not in another. For example, we can say (unusually perhaps) that we have unhinged the door, meaning we have removed its hinges, and we can equally use the positive to say the door is now hanging on its hinges. However, the word unhinged can also refer to someone's unbalanced state of mind, but in this case we can't use hinged to refer to their state of sanity.

Un- often takes a very specific negative role to distinguish from other negative prefixes. For example, inhuman means "brutal, monstrous," while unhuman means "not of human form, superhuman."  When used with adjectives, un- often has a sense distinct from that of non-. Non- refers to a set of things that are not in the category associated with the stem to which it is attached, whereas un- refers to properties which are not like those of the typical examples of the category. Thus non-military personnel are those who are not members of the military, whereas someone who is unmilitary is unlike a typical soldier in dress, habits, or attitudes.

It's interesting, then, to see how much potential havoc a simple two letter prefix can wreak. I can also understand the look of relief on the language student's face when she reached that epiphany which I mentioned at the start. She had achieved it through a simple device whereby a model of negative adjectives or nouns and negative verbs had been paired, and the students encouraged to flesh out the situation described, saying what happened next or what had led up to the situation.

There was an unusual silence
as he unlocked the door.

She untied the parcel
with unnecessary haste.

Unable to resist the temptation any longer
Samantha  unmasked the mysterious stranger.

Duncan didn't unnerve them at all and
they succumbed to the unthinkable.

They are then encouraged to make up their own pairs and share these with colleagues. (I made a list of possible un- words for them to use, but they could use any that they thought of.). It worked remarkably well, and enlightenment was soon upon them. I didn't like to interrupt their joy with the news that there was even more confusion to be found in the prefixes re-, in- and dis-. That could wait for another day.