English National Opera Guides are ideal companions to the opera. They provide stimulating introductory articles together with the complete text of each opera in English and the original. ''See how the drama is turned into laughter," comment the chorus,as they witness the destruction of a marriage and a friendship. Here Pierluigi Petrobelli shows how the irony woven into the tragedy results from the meeting of French and Italian traditions. Is this really a ridiculous libretto mangled by censorship,or is it Verdi's finest achievement,with its perfectly symmetrical drama and beautiful variety? Benedict Sarnaker analyses the extended love duet -the kernel of the score,and the pivot of the action. Harold Powers looks at the laughing chorus and at the quicksilver character of Oscar,unique in Verdi's work.