前缀a-来源混乱，形式复杂多变，需要认真甄别。
主要含义有以下四种：
1.表示否定：无，不，非。
prefix meaning "not, without," from Greek a-, an- "not" (the "alpha privative"), from PIE root *ne- "not" (source also of English un-).
来自希腊语前缀a-，缩写自an-：不，非，没有。其中an-用于元音前，其缩写形式a-用于辅音前。见前缀an-。
acentric=a-centric, 无-中心的
asocial=a-social, 不-好社交的
atypical=a-typical, 非-典型的
asymmetry=a-symmetry, 不-对称
adynamic=a-dynamic, 无-力的
aperiodic=a-periodic, 非-周期的
amoral=a-moral, 非-道德性的
asexual=a-sexual, 无-性别的
apolitical=a-political, 不-关心政治的
ahistorical=a-historical, 与历史无-关的
2、含有in,on,at,by,with,to,of等意义
asleep=a-sleep, 在-熟睡中
aside=a-side, 在-一边
abed=a-bed, 在-床上
ashore=a-shore, 向岸上，在-岸上
afield=a-field, 在-田里，在野外
aground=a-ground, 在-地面上，搁浅
ahead=a-head, 向前，在-前头
abreast=a-breast, 肩并肩地
afoot=a-foot, 徒步
atop=a-top, 在-顶上
afire=a-fire, 在-燃烧中
aback=a-back, 向后
asleep=a-sleep, 在-熟睡中
aside=a-side, 在-一边
abed=a-bed, 在-床上
ashore=a-shore, 向岸上，在-岸上
afield=a-field, 在-田里，在野外
aground=a-ground, 在-地面上，搁浅
ahead=a-head, 向前，在-前头
abreast=a-breast, 肩并肩地
afoot=a-foot, 徒步
atop=a-top, 在-顶上
afire=a-fire, 在-燃烧中
aback=a-back, 向后
3、表示加强
aloud=a-loud, 高声地
awake=a-wake, 唤醒，使醒
await=a-wait, 等待
arise=a-rise, 起来，升起
afar=a-far, 遥远地
aright=a-right, 正确地
aweary=a-weary, 疲倦的，厌倦的
ashamed=a-shamed, 羞耻的
alike=a-like, 相同的（地）
awash=a-wash, 被浪潮冲打的
etymology online 给出前缀a-的解释分为三个，如下:
1.
1）on;
2) off, from;
3）motion away from
4) to, toward; for" (see ad-)；
5）or ab "from, away, off" (see ab-);
prefix or inseparable particle, a relic of various Germanic and Latin elements.
In words derived from Old English, it commonly represents Old English an "on" (see on (prep.)), as in alive, above, asleep, aback, abroad, afoot, ashore, ahead, abed, aside, etc., forming adjectives and adverbs from nouns, with the notion "in, at; engaged in." In this use it is identical to a (2).
It also can represent Middle English of (prep.) "off, from," as in anew, afresh, akin, abreast. Or it can be a reduced form of the Old English past participle prefix ge-, as in aware.
Or it can be the Old English intensive a-, originally ar- (cognate with German er- and probably implying originally "motion away from"), as in abide, arise, awake, ashamed, marking a verb as momentary, a single event. Such words sometimes were refashioned in early modern English as though the prefix were Latin (accursed, allay, affright are examples).
In words from Romanic languages, often it represents reduced forms of Latin ad "to, toward; for" (see ad-), or ab "from, away, off" (see ab-); both of which by about 7c. had been reduced to a in the ancestor of Old French. In a few cases it represents Latin ex.
[I]t naturally happened that all these a- prefixes were at length confusedly lumped together in idea, and the resultant a- looked upon as vaguely intensive, rhetorical, euphonic, or even archaic, and wholly otiose. [OED]
2. away，
例词avert，avocation
word-forming element meaning "away," from Latin a "off, of, away from," the usual form of Latin ab before consonants (see ab-). As in avert, avocation. It is also the a in a priori and the à in Thomas à Kempis, Thomas à Becket.
3.
1）not, without，无、不、非；例词： abysmal, adamant, amethyst;
2）在希腊语里表示“一起”，例词：acolyte, acoustic, Adelphi；
prefix meaning "not, without," from Greek a-, an- "not" (the "alpha privative"), from PIE root *ne- "not" (source also of English un-).
In words from Greek, such as abysmal, adamant, amethyst; also partly nativized as a prefix of negation (asexual, amoral, agnostic). The ancient alpha privatum, denoting want or absence.
Greek also had an alpha copulativum, a- or ha-, expressing union or likeness, which is the a- expressing "together" in acolyte, acoustic, Adelphi, etc. It is from PIE root *sem- (1) "one; as one, together with."
,
