There is a pattern to help one decide if un- or in- is the correct prefix for a word. As with many English spelling problems, the pattern is from the history of the English language. Both un- prefixes are from Old English. Both in- prefixes, and their various spellings, are from French/Latin. So the simple answer is that common words of English origin use the un- prefix. Also words of Norse or German origin use un- as they are related languages. Fancy imported words of French or Latin origin use the in- prefix. That is why we have unseen, but invisible; unable, but incapacitated and untouchable, but intangible.

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Unfortunately as you have probably already guessed, English does not follow the pattern. So even if you knew the origin of a word, that will not guarantee a correct answer. For example inborn, inbred, indent and input are all words of Old English origin with the Latin/French in- prefix. Then there are Latin origin words using the English un- prefix. For example: unambiguous, uncontrolled and unemployment. However in most cases the prefix normally used follows the pattern. 

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The next problem is how to tell if a word is of Old English origin or French origin. That is, without checking an historical dictionary. Common, easier to spell words tend to be of English origin. These words were used by ordinary people. Fancier, more difficult to spell words are from French. The lords and ladies used these words. For example: unseen, unfatherly and untie are from Old English; invisible, non-paternal and denouement are from French/Latin. 

If you don't know which prefix to use check a dictionary. When that takes too long, use in- for longer difficult to spell words and un- for everything else. When in doubt use un-. Whatever is the most commonly used combination is generally correct.

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Related articles:

The Prefix Un-

The Prefix In-

How Does One Spell the In- Prefix?

by John Larrysson [email protected]

A native English speaker who has been teaching practical English in Hong Kong for over two decades.

John Larrysson's Column: link.mingpao.com/15488.htm

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