Are there any patterns to observe in choosing the correct negative prefix to use, as in unbelievable, disproportionate, asymmetric, and intolerable? (There are other negative prefixes as well, but these are the ones I usually mix up.)
Un- and in- are probably the ones I most frequently mix up, as in *untolerable/intolerable, *unedible/inedible, *unexact/inexact, *unappropriate/inappropriate, ... I realize these are all in-, but why? Is there anything to look for there?
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.
a- and an- are used in words with Greek origin, (meaning no, absence of, without, lack of, not in 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 ones). The rule of thumb is that, as previously stated:
un- is used in words of Germanic origin,
in- in words of Latin origin (and generally in words from other Latin-derived languages, like Spanish or French),
Prefixes like de-, dis- and dys- come from Latin or Greek and have a slightly different meaning.
Note that un-, in- and a-/an- have the same origin, but they have taken very different ways into our language of today.
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.
The general rule is to choose the prefix according to the etymology of the word:
However, the un- prefix is usually used for Latin derivatives that end in suffixes such as -ed and -able , resulting in adjectives such as unfounded, unassailable, and unbelievable. This to prevent confusion with another Latin prefix with identical appearance but a different meaning of "to cause to", which appears in words like illuminate, irradiate, or inflame.
Latin-derived words with un-:
Roots with different prefixes for adjective and noun forms:
Roots with both un- and in- variants with distinct connotations:
