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Consider the following words:

impossible;
incorrect;
impatient;
illegal;
irregular;

The meaning of the prefixes is the same (negation the adverb), but they are still different prefixes.

How do I know which one to use? In other words, for an arbitrary adjective, how do I choose between "in-", "im-", "il-", and "ir-"?

The relevant discussion at ELU does not provide with any guideline.

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  • Irregardless of these rules pay attention not to double the negative by using both a negative prefix and negative suffix.
    – SF.
    Commented Jan 29, 2013 at 8:41
  • @SF. correct, and also some stems are confusing: "important", "industrial"
    – Be Brave Be Like Ukraine
    Commented Jan 29, 2013 at 14:39
  • Also "en-" and "em-", especially but not exclusively in words derived from Latin via French, e.g. "inquire" versus "enquire".
    – user230
    Commented Feb 24, 2013 at 17:14
  • 2
    The meaning of the prefix is negation in your examples; but it can also mean in, as in innate, impersonate, illuminate and irradiate.
    – StoneyB on hiatus
    Commented Feb 24, 2013 at 20:49

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Formally, it is the same prefix "in-".
However, there are four different spellings that help pronunciation.

The rule only depends on the first letter of the adjective:

L — use "il-": illegal, illiterate;
P, M, or B — use "im-": improper, immediate, imbalance;
R — use "ir-": irresponsible, irrational;
other consonants or vowels — use "in-": indifferent, innumerable, inadequate;

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  • 1
    +1 It's possibly worth adding that these uses are all adopted from or extensions of the practice in Latin, from which the prefix is borrowed, along with many of the words.
    – StoneyB on hiatus
    Commented Jan 29, 2013 at 13:51
  • 5
    I've never heard (or seen) "innatural"; "unnatural" is what I would expect. Webster's 3rd Unabridged shows fewer than four columns of "in-n*" words, and fewer than five of those words are negative, "innumerable" being the most familiar. For most, the "in" supplies the sense of "inherent". (However, if an "in" prefix is appropriate -- for negation or innateness -- the "n" should definitely be doubled.)
    – barbara beeton
    Commented Jan 29, 2013 at 13:54

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