A reader asks about differences between the prefixes un- and dis-.
The question is not easy to address.
The prefix un- has been in the language longer than dis-.
The Old English prefix on- (now spelled un-) was added to verbs to indicated a reversal of the action:
This prefix has remained alive, giving us such verb opposites as:
Old English also had the prefix of negation un- that was added to adjectives, such as unborn and unburied. We continue to form negative adjectives in this way:
Dis- came into English during the Middle English period, along with many Latin and French words. The prefix dis- is related to bis, (two), and can be used in the sense of separation:
In the course of centuries, distinctions between un- and dis- have blurred. Sometimes the prefixes are interchangeable. Sometimes not.
Sometimes a perceived difference may exist only in the mind of the individual English speaker.
Many speakers distinguish between disorganized and unorganized.
Disorganized applies to the sort of person who stuffs receipts into the sock drawer and can never find the car keys. Unorganized applies to things which have not yet been arranged in an organized manner. By this reasoning, a person would be disorganized, but an office would be unorganized.
At one time, unease and disease (first syllable stressed) could be used interchangeably with the meaning “state of anxiety.” Now disease (second syllable stressed) has taken on the meaning of “illness.”
Angry arguments are waged over the differentiated meanings of uninterest and disinterest. The argument is that uninterested should be used with the sense of “indifferent, lacking in interest, while disinterested should be used only when the intended meaning is “impartial.” Some argue against the distinction on historical grounds, but the perceived difference in modern usage is a useful one.
When it comes to language, those who value logic above all else are just asking for elevated blood pressure.
For example, the noun discontent is matched with the adjective discontented, but the adjective that corresponds to the noun discomfort is uncomfortable.
About all one can safely say about the use of the prefixes un- and dis- is that their correct use is often a matter of idiom.
The best way to master them is to read, listen, and look up questionable forms in a trustworthy dictionary.