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Is the prefix im- used in a negative sense, as in, the opposite of the word following it, e.g.

  • Impenitent = "not penitent"

Or it is used in the positive sense that supports the word following

  • Impassive = more or less same meaning as passive.

When do I know if it is opposite and when not?

  • You know it by learning all words by heart. Just like everyone else, everywhere, in the entire history of the language. There is no other way. There can't be. – RegDwigнt Jun 20 '13 at 8:39
  • What are we to do with pertinent and impertinent? – Brian Hooper Jun 20 '13 at 15:25
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It should be noted that the prefix im- is really just the prefix in- with the consonant adjusted because of the consonant that follows.

As for passive and impassive, it's a quirk of the history of the meaning that they've ended up with similar meanings. Originally passive meant feeling pain and impassive meant not feeling pain. The former came to mean not active while the latter came to me without emotions.

But you're right, as other cases show, in- is a particularly difficult prefix to discern meaning from for words you haven't encountered before.

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    Especially because there are two prefixes in-, both from Latin. One in- means 'not', as in illiterate or irreverent (negative in- also assimilates before L and R), but the other in- means 'inside', like inhabit, internal, intromission. They were homophones in Latin, too, and English borrowed all those Latin words, so we got both prefixes too. – John Lawler Jun 20 '13 at 14:08

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