opera

Opera, operae, f., first declension, and the sister of the neuter opus, operis. Split them carefully: opus is the work done, the finished thing; opera is the effort put in, the trouble taken, the attention given.

The idioms are the point of the word. Operam dare alicui rei is "to devote effort to something", and da operam ut with a subjunctive is the ordinary way to tell someone to see to it that. Operae pretium est means "it is worth the trouble", literally "it is the price of the effort". In the plural operae can mean the workmen themselves, hired hands, and in political abuse a paid mob. One tangle worth knowing: Italian opera in the musical sense descends not from this noun but from the neuter plural of opus, reanalysed as a feminine singular.