What is the difference between the prefix 'in' and 'un' in English from a linguists point of view?

Answer 1

The present 'English' language, with its prefixes and suffixes, and all its forms and patterns, is the legacy of the past.

'in-' and 'un-' are examples of negative prefixes, though not all words that start with these prefixes are necessarily negative. (e.g. inflammable!) Therefore the usage of in- or un- is more a matter of custom and habit rather than there being some rule on the matter.

• The prefix in- /im- is Latin, and is usually used with Latin roots. e.g. incomplete, (not complete), impossible (not possible.)
• 'un-' is the native English prefix for negation, and is very widely used with all sorts of words and roots. e.g. uncertain (not certain), unwise (not wise.)

LINGUISTICALLY and semantically there is no real difference, if any, between in- or un-. Both are negative prefixes, they both 'negate' the meaning of the root word. Both prefixes meaning 'not' or 'the opposite of.' (Sometimes un- means the reversal of a process e.g. undo, unscrew)

THE AVAILABILITY of alternative negative prefix forms makes it very difficult for learners of the language to know which prefix to use with which root. There is no rule. The only way to know for certain is to check in a dictionary. Then, each 'negative' word has to be learned by itself, or, even better, as part of a group of words that all have the same prefix.
But once you know, you'll know! And it just won't sound right any other way!

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Answer 2

The prefix 'in' typically indicates negation or reversal, as seen in words like 'inactive' or 'incomplete.' On the other hand, the prefix 'un' usually conveys the idea of 'not' or 'opposite,' as in 'unhappy' or 'uncertain.' While both prefixes serve to change the meaning of a word, they do so in slightly different ways according to their linguistic roots and historical usage.