1. ai — Lewis & Short
ai, in old Lat., corresponding to ae: AIDILIS, CAISAR, AITERNOS, for Aedilis, Caesar, aeternus; also, still later, sometimes in the poets in the termination of the genitive of the first decl.; but, as in Enn. and Lucr.,
I per diaeresin always dissyl. with long penult:
furit intus aquāï,Verg. A. 7, 464:
aurāï simplicis ignem,id. ib. 6, 747:
terrāï frugiferāï,Mart. 11, 91, 5; cf. Quint. 1, 7, 18; Spauld. Prisc. 728; Prob. 1438; Vel. Long. 2222; Mart. Vict. 2460 P.—In prim. syllables, as in voc. Gaĭ, ăi could not be changed to ae if i was an ending; but i was changed to i cons., when the word received accession, e. g. Gaius.—When a conson. followed ai, as in CNAIVOS for *g*n*a*iv*o*s (v. the Epitaphs of the Scipios, in the Append.), ae was written at a later per., as Gnaeus; hence from *gra/i+os both Graecus and Graius; from *ai)/akos, Aeacus, and Aiax, for *ai)/as, were formed; just as Achaeus or Achivus with Achaĭus or Achaĭcus was used.
2 * ai = ai)/, interj., denoting grief, ah! alas! Ov. M. 10, 215.