The Ablative with opus est is usually explained as a relic of the Instrumental Case, e.g. opus est gladio ‘there is a work (to be done) with a sword’1 To the Agent the Genitive would be as appropriate as the Ablative to the Instrument, e.g. Most. 412 “id viri doctist opus”. We find the Genitive of the thing in Lucilius 334 Ma. “nummi opus” (see Marx's note). In Plautus we find also the Nominative, e.g. Capt. 164 “opus Turdetanis, opust Ficedulensibus, iam maritumi omnes milites opus sunt tibi”; but whether the Grammarian Nonius Marcellus (482 Me.) is right in saying that the Accusative was also used is doubtful. Cf.
- Truc. 88 (of uncertain text),
- 902 “puero opust cibum” (-bo, edd.),
- Ter. Phorm. 666 “opus est sumptu (-tum A） ad nuptias.”