opus

Opus, operis is third declension NEUTER, so nominative and accusative are opus and the plural is opera. Do not let that plural be confused with opera, operae, a separate first declension FEMININE noun meaning effort or trouble, the one in operam dare, "to take pains". Two different words that meet in one spelling.

The construction worth drilling is opus est, "there is need". The person who needs goes in the DATIVE and the thing needed usually in the ABLATIVE, so mihi pecunia opus est is literally "for me there is need by money" and means "I need money". It reads backwards to an English eye and it is extremely common. The thing needed can also appear in the nominative, treated as the subject, which is why quid opus est verbis puts verbis in the ablative while nihil opus est puts nihil in the nominative.

Concretely, opus is a work in the sense of a made thing: a building, a fortification, a book. Opus est facto is "it needs doing". English took opus, opera, operate, operation and, through manu operari, manoeuvre.