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Are there any patterns to observe in choosing the correct prefix to use? There are other prefixes as well, but these are usually the ones I mix up.

As in unbelievable, disproportionate, asymmetric, and intolerable.

Un and In are probably the ones I more most frequently mix. Is there anything to look for there? Like untolerable/intolerable, unedible/inedible, unexact/inexact, unappropriate/inappropriate, ...

I realize these are all in-, but why?

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up vote 8 down vote accepted

Agreeing with the answer Shinto Sherlock gave from Fowler, World Wide Words says,

In general, words take un- when they are of English (Germanic) origin and in- if they come from Latin. (The forms im-, il-, and ir- are variations on in-.) Apart from that, there’s really no good guide to which one you should choose.

Following the link will give you an expanded discussion of this topic.

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wasn't a- also for Latin / French derived words? – Abel Sep 5 '10 at 11:57
Hi Daniel, u mentioned about a link in your last statement. Where is the link? – Part Timer Aug 28 '12 at 8:29
@PartTimer the link is above the quote or here is the url: worldwidewords.org/qa/qa-unv1.htm. – Daniel Aug 28 '12 at 12:32

I'd like to put this in a comment, but since I cannot yet do that (low rep), I will write this as an answer:

a- and an- are used in words with Greek origin, (meaning no, absence of, without, lack of, not on said language). Other greek prefixes are ana-, (meaning again, up, on, above, c.f. anabaptist literally meaning "re-baptist") and anti- (meaning against, c.f. antidemocrat).

Anyway, the thing is that as language is a living thing with lots and lots of users (especially in the case of English, where there also are a lot of different cultures, further contributing to the confusion), and therefore, rules like this tend to be mixed up (especially when creating new words or combining already existing). The rule of thumb is that, as previously stated:

  • un- is used in words with German origin,

  • in- in words with Latin origin (and generally in words from other Latin derived languages, like Spanish or French),

  • a- and an- is used in words with Greek origin.

Prefixes like de-, dis- and dys- come from Latin and have slight a different meaning.

Note that most of these have the same origin, but they have taken very different ways in to our language of today.

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Fowler says:

The general principle that un- is English and belongs to English words, and in- is Latin and belongs to Latin words, does not take us far.

There is a lot more which you can read in Google books.

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