How would you explain to someone when to use the prefix "un" versus "in"?

For example:
inescapable
versus
unacceptable

An English Language Learner would certainly be confused...

Thanks!

Sorry, I can't help you here. I can, however, throw this in the mix:

"Inflammable means flammable? What a country!"

(Thanks to Dr Nick on The Simpsons for this one.)

Here's a bit more on prefixes.