ad- (Latin: to, a direction toward, addition to, near; used as a prefix).
ad- appears before vowels and before the consonants d, h, j, m, and v:
Before sc, sp, and st; ad- is simplified to a-: ascend, aspect, et al.
Before c-, ad- is assimilated to ac-: accelerate, accept, accept, accident, accord, accumulate, accurate, accurate, et al.
Before f, ad- becomes af-: affable, affect, affidavit, affiliate, affinity, affirm, affix, afflatus, afflict, affluence, et al.
Before g, ad- becomes ag-: agglomeration, agglutinate, aggrandize, aggravate, aggravate, aggregate, aggression, aggressive, aggressive, aggressor, et al.
Before l, ad- becomes al-: allege, allegiance, alleviate, alliteration, allocate, allude, allure, allusion, alluvium, et al.
Before n, ad- becomes an-: annex, annihilate, annotate, announce, annul, annulment, et al.
Before p, ad- becomes ap-: apparatus, appeal, appearance, append, appendage, appendix, appetite, applaud, applause, applicable, application, applied, apply, appoint, apportion, apposition, appraise, appreciate, apprehend, apprentice, approach, appropriate, approve, approximate, et al.
Before q, ad- becomes ac-: acquaint, acquaintance, acquiesce, acquiescence, acquire, acquisition, acquital, et al.
Before r, ad- becomes ar-: arrears, arrest, arrive, arrogant, et al.
Before s, ad- becomes as-: ascent, ascertain, ascribe, aspect, aspiration, assail, assailant, assault, assemble, assent, assert, assertion, assertive, asset, assiduous, assign, assignment, assist, assistant, associate, assonance, assortment, assuage, assume, assumption, assurance, assure, assuredly, et al.