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I took the ESL certification test and I was wondering: what is it called when you add 'im' to 'possible'?

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Thrice didn't cut it for me :( I'm still lost. – Ward Muylaert Jan 30 '11 at 16:13
Four times was not enough for me either. What does was what is it called when you add 'im' to 'possible' mean? I read it as How is the process of forming words like 'impossible' called? – kiamlaluno Jan 30 '11 at 16:27
It's called defeatism! :) – Robusto Jan 30 '11 at 16:32
@Robusto: I LOL'ed. – Potatoswatter Jan 31 '11 at 10:07

It's funny because you tagged this question prefixes — well, adding im- to possible is called prefixation.

(Making a word have the opposite meaning by adding a negating prefix is also a form of negation.)

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I didn't even notice the tag. Good one. – Robusto Jan 30 '11 at 16:33
Found morphophonemic alternation - which I guess has to do with the im rather than in :) – mplungjan Jan 30 '11 at 17:26
@RegDwight: My initial assumption was that the OP was asking for negation. – oosterwal Jan 31 '11 at 19:42

Adding a prefix to a word is called prefixation, as Kosmonaut pointed out.

A further point: as several dictionaries say, impossible comes from the prefix in- and possible, so why is in + possible = impossible, rather than *inpossible?

The answer is that in- becomes im- before words starting with 'p', 'b', or 'm' (as in impatient, imbalance, immature, etc.), because of a process of euphonic sound change known in Sanskrit as sandhi (pron. "sun-thee"), a term that appears to have some currency among general linguists as well. Specifically, it's a form of internal sandhi (the kind that would be called parasavarṇa sandhi in Sanskrit, but I don't know the equivalent term in English linguistics).

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@Jasper: Is assimilation the term for sandhi in general, or for consonant sandhi, or for the specific type of sandhi where -n becomes -m before p/b/m? (And becomes the velar nasal before velar consonants, etc., which are not reflected in spelling.) – ShreevatsaR Jan 31 '11 at 8:14
Outside Sanskrit studies, the word "sandhi" tends to be used mainly for processes at the juncture of separate words; but this might be called "internal sandhi". "Assimilation" is the kind of change - one sound taking on the characteristics of a neighbouring sound - and most often applies where separate morphemes come together, but can apply in other circumstances. For example "bank" is pronounced with the 'ng' sound of "bang" because of the following velra. – Colin Fine Jan 31 '11 at 18:28
@Colin: Thanks, I guess what we have here (and in your examples, and what I was looking for) is assimilation, specifically nasal assimilation. – ShreevatsaR Feb 11 '11 at 20:36

To add a prefix to a word is called prefixation, as Kosmonaut points out.

To add any kind of affix (prefix, suffix, infix, circumfix) to a word is called affixation.

To form a new word out of another word by means of affixation is called derivation or sometimes agglutination.

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The semantic process (effect on the meaning) is called "negation" or "privation".

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