Given the different questions we have seen about the prefixes "de-" and "un-", I have grown curious if there is a overarching rule for terms that need undoing.
For actions taken that need to be revoked or reversed, which prefix is preferred? Is there a distinction created by the underlying terms? Is it purely a usage issue where the one most used is the one to use? Do "un-" and "de-" actually have different meanings such that both could help determine which is most appropriate?
Some prescriptive grammarians would argue that the de- prefix should be used on verbs and un- should be used on adjectives.
So, you deregister something and it becomes unregistered, or you deselect something and it is now unselected.
The logic behind this is probably because de- only attaches to verbs to give the notion of reversal, so for the sake of order/non-redundancy/etc. you'd want un- to occupy the other domain: yielding the opposite meaning of an adjective it attaches to, and nothing else.
However, it has never really worked this way; the prefix redundancy among verbs is there, and it is very unlikely to disappear. Note that, most of the time, you can't just use whichever one you want — usually there is one preferred form. But whether it is un- or de- is something that varies on a word-by-word basis.
FumbleFingers mentioned something else in comments that is worth mentioning: un- seems to be much more widespread, even in the verbal realm, in the production of new words in the past few decades.
Very broadly, de- is more likely to indicate action, whereas un- is more likely to connote a passive status: After you detune your guitar, it is an untuned guitar.
In practice, of course, there are so many exceptions and counterexamples as to render this "rule" largely meaningless. I would guess, though, that people are somewhat more likely to follow this convention than the opposite when coining ad-hoc new words from existing roots (example: to deselect a check box in a computer GUI is an action, whereas unselected is a status that the box can have).
Among verbs, I believe that un- is used to intrinsically undo something (eg, uncreating an object) whereas de- means to reverse its effects (eg, decompiling a program), without modifying the original item.
One might want to unsay something—to take back the fact that it was said in the first place.
People become desensitized to things—their sensitivity is nullified.
People become demotivated—their motivation is reversed.
People unsubscribe from email lists—they get rid of their subscription
People unwrap things—they get rid of the original wrapping
Arguments are deconstructed—they are broken down and attacked; the argument itself is not destroyed.
