The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
411. Opus and usus, signifying neet', take the
Ablative: [This construction is properly an instrumental one, in which
epus and Usus mean work and service, and the ablative expresses that with
which the work Is performed or the service rendered. The noun usus
follows the analogy of me verb titer, and me Lblative wim OPU8 C8t appeals
to be an extension of that with 58U8 est.]
- magistrAtibus opus est (Leg. iii. 5), there is need of magistrates.
- nuno viribus lens (Aen. viii. 441), now there is need of strength.
NOTE: The ablative with tisus is not common in classic prose.
a. With opua the aNative of a perfect
participle is often found, either agreeing with a noun or used as a neuter
abstract noun :
- opus est tuA expremptA malitiA atque
astititt (Tar. And. 723), I must ha"s your best cunning and
cleverness set to work.
- properdto opus erat (at. Mu. 49),
there was need of haste.
NOTE 1: So rarely with Usus in comedy: as, - quid istis tisust canseriptia (Pl. Bacch. 749),
what's the good of having them in writing?
NOTE 2: The omission of the noun gives rise to complex
constructions: as, - quid opus factist (ef. B. 0. i. 42), what must be done? [Cf. quid epus eat fleri? with quo* Rct5 opus est?]
b. Opus is often found in the predicate,
with the tMng needed 'a the nominative as subject:
- dux ne-bis et auctor opus est
(Fam. ii. 6.4), we need a chief and responsible adviser (a chief, etc., is necessary for us).
- Si quid ipsi opus asset (B. G. i. 84), if he himself wanted anything (if any. thing should be necessary for him).
- qua. opus sunt (Cato R. R. 14.8), things which are required.