The English prefix un- comes directly from Old English (and shares a common Indo-European root with Latin in- and Greek a-) Prefixed to an adjective A, the resulting new word means ‘not A’ and can convey either positive or negative sentiment, depending on the meaning of the adjective.

• selfish/unselfish, pretentious/unpretentious  (un- has positive sentiment)
• happy/unhappy, flattering/unflattering (un- has negative sentiment)
• usual/unusual, aware/unaware (un- context-dependent sentiment)

Prefixed to a verb, un- conveys the reversal of a process:

• load/unload
• tangle/untangle
• wind/unwind
• do/undo

However, not all verbs form their opposite by adding un- as a prefix. For example

• break/*unbreak

You can’t unbreak a teapot, you can only repair it. (Perhaps the special context of watching a film in reverse would lend itself to such a usage ‘watch the teapot unbreak in frame 254.’ ??)   Notice that there is symmetry for the associated adjectives — an object is either in a state of being broken or intact, or has the potential to be in one of these states.

• broken/unbroken
• breakable/unbreakable

Other examples of verbs that are not symmetrical with respect to un- include

• clean/*unclean
• develop/*undevelop
• squander/*unsquander
• release/*unrelease

You must dirty, retard, save, or retain. Similarly, verbs that denote speech acts are not symmetrical with respect to un-.

• promise/*unpromise
• announce/*unannounce
• insult/*uninsult
• marry/*unmarry

You can only renege, retract, apologize or divorce. Verbs that describe mental processes also don’t have corresponding opposites formed with un-.

• remember/*unremember
• dream/*undream
• see/*unsee

The natural opposite of remember is forget, and there is the related misremember which implies recall of something not factual. (More on mis- in a moment.) Dream is more difficult; perhaps the opposite of dreaming is realizing? There seems to be no natural concept corresponding to a reversal or opposite of the event of seeing.  In summary, it appears that English uses un- only with verbs that describe events which are simple, reversible physical processes, seemingly ignoring the arrow of time (even though each act of covering/uncovering, folding/unfolding, winding/unwinding is indeed moving in one direction through time).

But what about the prefix dis-?  Its occurrence with verbs doesn’t appear to conform to the above patterns. The following symmetrical pairs include both speech act verbs and mental process verbs.

• allow/disallow
• like/dislike
• please/displease
• regard/disregard
• invite/disinvite

Allow, please, regard and invite entered English from Old French, along with the Latin-derived prefix dis- which meant ‘not’. Dislike, however, is a hybrid form that replaced the native English mislike, which was at one time the opposite of like. The native English prefix mis- meant ‘wrongly, in error’ and we see it today in the verbs misjudge, misremember and probably mistake.

• I took him for an honest man
• I mistook him for an honest man

• please/displease/*unplease
• pleased/displeased/?unpleased
• pleasant/*displeasant/unpleasant
• pleasure/displeasure/*unpleasure