I would say we can start from a very well-known opera, something of an emblem of Italian Romanticism, "Lucia di Lammermoor" by Gaetano Donizetti.
I found the link to a complete performance on YouTube, which was broadcast at the time on Rai,
http://www.youtube.com/watch?v=dTgqPzKOH-I
here is the libretto online
http://www.librettid...m/lucialam.html
surely you can also find downloadable versions if you look, but it's up to you.
and if you are very motivated, here is the link to download complete scores and vocal-piano scores
http://imslp.org/wik...zetti,_Gaetano)
The original division of the opera is a bit complex, but let's say it consists of three acts, each divided into two scenes (the third has a sort of introductory scene). The story is somewhat similar to that of Juliet and Romeo: Lucia Ashton is in love with Edgardo Ravenswood. The problem is that the Ashtons destroyed the Ravenswood house; Edgardo deeply hates Lucia's brother, Enrico. Edgardo leaves to fight with his faction in the various Scottish struggles of the period (we are in an unspecified 16th century); Enrico, who is on the opposing side, arranges a marriage of interest for Lucia with Arturo Bucklaw (affectionately called "the little prince"). Lucia does not want to, but eventually gives in, and as soon as she signs the marriage contract, Edgardo bursts in, discovers her, and vows revenge. The duel between Enrico and Edgardo does not actually take place, but on the first wedding night, Lucia goes mad, kills Arturo, and commits suicide. Edgardo, unaware of everything, speaks to the tombs of his ancestors while contemplating suicide... and as soon as he learns that Lucia has gone mad, he kills himself.
The story is taken from the novel by Walter Scott, The Bride of Lammermoor.
It would be nice if you tried to watch the whole opera.
Anyway, like all 19th-century operas, this is a "number opera." This means that every moment can be identified as a form, or a series of forms, so it is countable.
The typical division is
1) scena (meaning the beginning of a new scene, with new characters, new actions)
2) recitativo (one or more characters introduce the subject in a vocal style between singing and speaking)
3) tempo d'attacco of "something," meaning a closed piece, whether it be an aria, a duet, or a trio
4) cantabile (the central part of the closed piece)
5) tempo di mezzo (anything can happen here, but let's say it is the connecting moment between the two sections of the closed piece)
6) caballetta (the final and generally fast part of the closed piece)
Anything that deviates from this division is considered innovative.
Practically the entire opera, then, is made up of precise scenes and, above all, recognizable closed pieces.
The most famous and "important" closed pieces of Lucia di Lammermoor are
"Cruda funesta smania", Cavatina (meaning the presentation aria of a character) by Enrico (first act, scene one)
"Regnava nel silenzio", cavatina by Lucia (first act, scene two)
"Sulla tomba che rinserra", duet Lucia-Edgardo
"Il pallor funesto orrendo", duet Enrico-Lucia (second act, scene one)
"Chi mi frena in tal momento", sextet (second act, scene two)
"Il dolce suono mi colpì di sua voce", the great aria also known as Lucia's mad scene (third act, scene two)
"Tombe degli avi miei", aria by Edgardo (third act, scene three)
If you search for them like this, with these titles, you will find many on YouTube. So, if you don't feel like approaching the listening of the entire opera right away, you can also "get a taste" with these individual pieces.