A6. Although the above question appears to be simple, it requires answers to
specific parts in it, as follows:
A6.1. "What it the rule concerning prefixes that turn words into their opposites?"
The meaning of the words is studied by a grammatical branch named "Semantics". Unfortunately, semantic topics are way too
complex to present in one, beginner-level grammar book; therefore, they are presented/exemplified only marginally and incidentally
in Logically Structured English Grammar 4. However,
in LSEG4 there are subchapters/sections dedicated to the most common methods used to form each morphologic element--including
Now, one common method of forming new, custom-built morphologic elements (using derivation) is named "affixation": this is,
using prefixes and suffixes added to a "root-word". This affixation process was done a long time ago for most
words, though it still continues today using general-meaning prefixes and suffixes. In this last instance, the result is
custom-built morphologic elements.
English was formed as a mixture of many languages: the original Beaker-culture basis/fundament, plus Latin, Greek, German,
Scandinavian, French, Hebrew, Spanish, Italian, Russian, etc. That particular aspect has generated a bit of a chaos in structuring
the English morphologic elements based on strict grammatical rules.
For example, some morphologic elements (adjectives, nouns, adverbs) take particular and specific sets of prefixes/suffixes.
LSEG4 presents (partially) the most frequent sets of prefixes/suffixes used to form the morphologic elements (including the most
frequent instances of opposite/negative meanings). In addition, LSEG4 presents a few sets of common, general-meaning prefixes used
to form new, custom-built morphologic elements.
A6.2 "When do I use (un-) unpleasant, (in-) incapable, (im-)
impossible, (a-) atheist, and (non-) nonbeliever?"
First of all, words are grouped into:
A. well known words, and we rely on dictionaries to use them appropriately;
custom words/(equivalent morphologic elements), and we need to exercise great caution when forming/using them.
The set of words submitted for analysis are all adjectives, therefore they take adjective-specific sets of
prefixes. Particular to adjectives is, some prefixes and suffixes are indeed used to form opposite/negative meanings. For
example, the opposite/negative meaning prefix:
a. (un-) is of German origin, and it is used a lot (uncommon, unknown, untold);
b. (in-) is of Latin origin, also quite frequent (inadequate,
c. (im-) is of Latin origin (immaterial,
d. (a-) is of Latin origin when it is used with its opposite/negative meaning (amoral, aphonic, apolitical), and of German origin in all other instances (aflame, ablaze, aglow, ahead, alike)
e. (non-) is of Latin origin. From the list of prefixes submitted, (non-) is the only general-meaning prefix that may be
used (safely) to form custom negative-meaning adjectives. Note that in most instances (non-) is tied to the root-word
using a hyphen/dash (non-conformist, non-aromatic, non-Catholic) particularly because it
is a general-meaning prefix.
In LSEG4 the readers can discover a few more (recommended) general-meaning prefixes: anti-;
extra-; retro-; super-; ultra-; etc.
Using one known prefixed word/adjective for another is a tough choice (amoral / immoral), and please be aware that each
version/form may have elusive, particular or additional meanings. Our recommendation is, use a few good dictionaries issued in USA,
and in UK--sometimes there are significant semantic differences between the two; when this happens, the USA ones appear to be
far more reliable on technical/scientific aspects.
Forming new adjectives using general meaning prefixes is a practice we would like to see discouraged. However, if you do
that, please mark your new adjectives appropriately using a hyphen/dash [of course, without colors, italics, or bold type]:
ex-military, contra-nature, non-human, etc.
Please be aware the addition of prefixes/suffixes may come, sometimes, with orthographical changes to the
root-word, required by phonetic-agreement.