Discussion Forum—A Way with Words, a fun radio show and podcast about language

Oh, how I wish I could provide you with guidelines, but, to my knowledge, such guidelines do not exist. I was an ESL teacher for years, and I had looked into that same question a bit. And what I came up with was probably the same as what you've come up with: nothing. As far as I can tell, those negative prefixes are one of those special areas that make English so remarkably difficult for speakers of other languages. Those prefixes and the words they attach to often don't have even internal logic; for example, uninterested and disinterested mean different things, although the prefixes un- and dis- both have negative influences on their respective roots.

In short (a little late for that now), English affixes are often illogical, and, much like irregular past tense verbs and past participles, there is no simple instruction method. The words simply (simply?) must be memorized. I'll say, though, your students probably can't go wrong if they just stick to using the word not instead of negative prefixes.