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This question already has an answer here:

adverbs like inefficient, inexpensive, unnbelievable..., imbossible

Is there any roule? When to use? What to do

marked as duplicate by sumelic, Cascabel, jimm101, JJJ, user067531 Jul 11 '18 at 22:11

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  • Prefixes in-, im-, ir-, il- are all forms of the same thing, which to use depends on the beginning of the following word. Of course un- is different. – GEdgar Jul 11 '18 at 17:04
  • In addition to the question I linked to in my previous comment, you may also want to look at some of the other questions with the tag negative-prefixes – sumelic Jul 11 '18 at 17:05
  • There’s no good rule that is easy enough to remember that doesn’t have so many exceptions to also remember that it’s worthwhile to remember the rule instead of remembering the individual words, or using a dictionary when you forget. – Dan Bron Jul 11 '18 at 17:05
  • Anywhere but in front of "flammable". – Hot Licks Jul 11 '18 at 17:18
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There isn't really a rule of thumb for these. At the very least, you could say that im- is used when the adjective starts with a consonant and in- when it starts with a vowel, but that doesn't help with un-. So I suggest you memorize which prefix is correct for each case.

  • In particular, "im-" is only used before the consonants M, P and B. – sumelic Jul 11 '18 at 17:27
  • @sumelic - Actually, "imflammable" was once a word. – Hot Licks Jul 12 '18 at 1:21
  • @HotLicks: Was it? The OED doesn't seem to have an entry for that spelling, and it doesn't list it as a variant spelling of "inflammable". I also can't see much use of "imflammable" relative to "inflammable" in any time period on the Google Ngram Viewer. Why do you say it was once a word? – sumelic Jul 12 '18 at 1:28
  • @sumelic - Back ca 1960 there was a hubbub in the US shipping and transportation arena because many people took "inflammable" (as used on, eg, tanker trucks) to mean "non-flammable". So "imflammable" was promoted as an alternative less likely to be subject to this confusion, and for a few years you'd see "imflammable" on tanker trucks, et al. But this of course caused even more confusion. (Does it then mean that "inflammable" does mean "non-flammable?) So they switched to using "flammable" and "non-flammable". – Hot Licks Jul 12 '18 at 1:39
  • @HotLicks: Wow, that's really interesting! I just asked a separate question about it: english.stackexchange.com/questions/454353/… In any case, now that I think about it, since that would be an example where "im-" is not negative, the rule I mentioned still holds in the context of negative prefixes. – sumelic Jul 12 '18 at 2:10

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