prefix
denoting motion or direction to
表示“往”，“向…”
advance.
adduce.
■reduction or change into
表示“减少”，“变成”
adapt.
adulterate.
■addition, increase, or intensification
表示“添加”，“增长”，“加剧”
adjunct.

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语源
from Latin ad 'to'; in the 16th cent. the use of ad- and its variants was extended to replace a- from a different origin such as Latin ab- (e.g. advance, from French avancer based on late Latin abante 'in front')

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用法
Ad- is also found assimilated in the following forms: a- before sc sp, st ; ac- before c, k, q ; af- before f ; ag- before g ; al- before l ; an- before n ; ap- before p ; ar- before r ; as- before s ; at- before t