Why do we go from using the prefix un- in unstable to using the prefix in- in instability? Why can't we just keep using un-?

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    You might find the accepted answer on this related ELU question to be helpful. – J.R. Apr 16 '15 at 8:30
  • very helpful. Academic words mostly came from Latin, which use in- as prefix, whereas commonly used words have a prefix of un- – Ben Zhang Apr 24 '15 at 0:55

According to http://www.worldwidewords.org/qa/qa-unv1.htm words with a Germanic root use the un- prefix and words with a Romantic root use the in- suffix.

There are also the prefixes a- as in asexual and non- as in nondescript.

Unless you're a professor of linguistics and etymology I think your best bet is to use un-, which is the most common prefix, and learn which words use a different prefix on a case by case basis.

EDIT: I know this doesn't completely answer your question for why unstable turns into instability, but the truth is that although there is a general rule for which prefix to use, there are so many exceptions that it's pretty much arbitrary (without reason).

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