My question was that,why people use untouchable not intouchable, why non-terminating not interminating?
The reason that these prefixes are different is because of the influence of other languages on the English Language. Take a look at the table and text below.
|a- (an-)||Greek||not, without||medium|
|dis-||Latin||not, opposite of||low|
|in- (il-, im-, ir-)||Latin||not, opposite of||medium|
|un-||Germanic||not, opposite of||high|
The prefixes a- and anti- are both of Greek origin. This explains why the very majority of the words that can use these prefixes also have their origin from Greek (e.g. chromatic, morphous, symmetric, typical, aerobic, hydrous, oxic).
The prefixes dis-, in-, and non- are Latinate in origin. Similarly, words that go along with dis- and in- are mostly from Latin/French. They include words like dishonest, discourteous, dissimiliar, inaccessible, inaccurate, insignificant, to name a few. On the other hand, while non- is also commonly associated with Latinate/French words such as non-negotiable, non-judgmental and non-specific, it has become more productive than the other Latinate prefixes. This prefix can form negative adjectives with many present participles and past participles regardless of the origin of the stem word, such as non-smoking, non-aligned, non-caffeinated, and even with participle phrases, such as non-profit making, non-man made. Another interesting fact about non- is that it can often form neagtive adjectives by joining verbs, to express the meaning that the thing described does not perform the action described by the verb. Examples include non-stop, non-shrink, non-slip
Un- is a prefix native to English. It is mostly attached to native words to form negative adjectives, such as unfriendly, unhappy, unfair, and so on. But it can also be attached to certain Latinate words, giving unable, unsympathetic, unconscious, unreasonable, etc. Like non-, it is a productive prefix and is ready to form adjectives with present and past participles, giving words like unfeeling, undecided, unjustified, etc.