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In English, we can use a number of prefixes on adjectives to form their opposites. The following pairs come to mind as examples:

In most cases, these prefixes are not interchangeable, so we do not have *unlegal or *apossible, for instance. Of course, fluent speakers have no problem in using the right prefix for the right word, as more than often, the negative adjectives are learned word by word. But that should not stop us from pausing and asking, “Are these prefixes randomly matched with the words?” The lesson from Why is probible not possible and possable not probable? tells us that while not everything has a reason, most things indeed do not come from nothing. So the question that we should really ask is “What are the differences between these prefixes?”

Intuitively, some (two groups) of these prefixes are obviously related, and indeed they are. The first group consists of a- and an-, and the second consists of il-, im-, in- and ir-. In each group, the variations have the same meaning, but are superficially different only because the initial sounds in the stem words are different. They are said to be phonologically conditioned. In the former group, an- is used when the following adjective begins with a vowel or an h, whereas a- is used otherwise. This can be illustrated by the following examples:

The latter group also follows very similar conditions. You may find out exactly how they are conditioned by listing out many words that start with these prefixes, and analyze how the following sounds affect them.

Having recognized that some of the prefixes are in fact no more than variations of the same prefix, our list of negative prefixes can be reduced to the following:

1. a- (an-)
2. anti-
3. dis-
4. in- (il-, im-, ir-)
5. non-
6. un-

We can survey these prefixes on three dimensions, namely their etymology (origin), semantics (meaning) and morphology (what words can the prefixes be used with, and how productive they are).

Etymology

Tracing back to their histories, the prefixes a- and anti- are both of Greek origin. This also explains why the very majority of the words that can use these prefixes also have their origin from Greek (e.g. chromatic, morphous, symmetric, typical, aerobic, hydrous, oxic).

The prefixes dis-, in-, and non- are Latinate in origin. Similarly, words that go along with dis- and in- are mostly from Latin/French. They include words like dishonest, discourteous, dissimiliar, inaccessible, inaccurate, insignificant, to name a few. On the other hand, while non- is also commonly associated with Latinate/French words such as non-negotiable, non-judgmental and non-specific, it has become more productive than the other Latinate prefixes. This prefix can form negative adjectives with many present participles and past participles regardless of the origin of the stem word, such as non-smoking, non-aligned, non-caffeinated, and even with participle phrases, such as non-profit making, non-man made. Another interesting fact about non- is that it can often form neagtive adjectives by joining verbs, to express the meaning that the thing described does not perform the action described by the verb. Examples include non-stop, non-shrink, non-slip.

Un- is a prefix native to English. It is mostly attached to native words to form negative adjectives, such as unfriendly, unhappy, unfair, and so on. But it can also be attached to certain Latinate words, giving unable, unsympathetic, unconscious, unreasonable, etc. Like non-, it is a productive prefix and is ready to form adjectives with present and past participles, giving words like unfeeling, undecided, unjustified, etc.

Semantics

Even though we may simplistically say all the prefixes carry a “negative, opposite” force, there are in fact subtle differences between their meanings.

Most obviously, anti- does not merely mean “not”, but “against”. So if someone is antisexist, it is not just the case that the person is not sexist, but he or she is in fact against sexism.

The Greek prefix a-, on the other hand, carries a greater sense of “without”, “lack of” and is therefore often used with adjectives which originate from nouns. The words chromatic (color), morphous (form), symmetric (symmetry), typical (impression), aerobic (air), hydrous (water), oxic (oxygen) all fit into this description.

The three prefixes dis-, in- and un- are very similar in meaning. They often denote a state on the opposite end of the scale. Therefore if somebody is impolite, he is not just being “not polite”, but in fact “rude”. If something is untrue, it is false. They are distinguished from non-, which expresses a simple negation. If something is nonreligious, it is not necessarily antireligious or irreligious, but just has nothing to do with religion.

As un- is a very productive negating prefix in English, it actually covers a wider range of meanings than other prefixes. Many words can form negations with both non- and un-, which gives near synonymous pairs like nonreligious / unreligious, nonproductive / unproductive, and so on. But quoting American Heritage Dictionary, they differ in that:

Non- picks out the set of things that are not in the category denoted by the stem to which it is attached, whereas un- picks out properties unlike those of the typical examples of the category. Thus nonmilitary personnel are those who are not members of the military, whereas someone who is unmilitary is unlike a typical soldier in dress, habits, or attitudes.

Morphology

As discussed above, what words can go along with a certain prefix has a lot to do with the etymology of that prefix. The prefix a- is more than often only used with Greek stems, and the stems more than often are adjectives that describe the properties of certain objects (nouns). This restriction makes the prefix less productive in Modern English, as apart from technical terms, new words are seldom coined from Greek.

Anti- is very productive and particularly so with adjectives that describe ideologies and attitudes, the majority of which are formed with the suffix -ist, such as sexist, racist, communist, capitalist, theist, so on and so forth. It can also be attached to any noun to form an adjective, such as anti-Microsoft, anti-government, etc.

Dis- is neither productive in modern English nor does it occur in a lot of words. Some adjectives that are seemingly formed by attaching the adjectival prefix dis- are in fact formed in a different process. For instance, disoriented is actually a past participle of the verb disorient, which consists of the verbal prefix dis- and the verb stem orient.

The prefix in- appears in a lot of words of Latin or French origins, but it is usually restrained to these existing words. Non-, as illustrated above, is rather productive and can be used variously to form negative adjectives.

The native prefix un- is probably the most productive of all in the above list. It can be used with participles and even participle phrases like unheard-of. And when new words are coined, it is usually the preferred prefix, thus we have ungooglable and unhackable. In fact, it actually took over the role of in- as the default negative prefix a long time ago. Since around the Renaissance, many words of Latin, French or Greek origins have formed their negative forms with un-, giving adjectives like unconscious, unconditional, unfortunate, unsympathetic, and so forth. This also explains those pairs which use different prefixes in the nominal and the adjectival counterparts, like inability / unable, inequality / unequal, injustice / unjust, and instability / unstable. In each of these pairs, the noun existed in Latin and was simply borrowed into English, whereas the negative adjective was later formed by attaching the prefix un- to the existing adjectival form. (cf. French inégalité / inégale, injustice / injuste, instabilité / instable)