ad-
/æd/prefix
1- denoting motion or direction to表示“往”, “向…”：
advance
adduce.
1.1
- reduction or change into表示“减少”, “变成”：
adapt
adulterate.
1.2
- addition, increase, or intensification表示“添加”, “增长”, “加剧”：
adjunct.
USAGE
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.
词源
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'）.