This piece is in continuous variation form and is in e minor.
The basis for this continuous variation is a melody in the bass line which is extremely simple with quarter notes that descend chromatically from the tonic tone of e down to the b where a half cadence is usually reached at the end of the phrase in the fourth measure. The orchestra plays half notes on top of this which establishes the chord progression, which is not constant for each variation.
The first variation of this line is marked by introduction of the voices who from the soprano to the bass do a little one measure motive which the other voices imitate at other pitches. The chords are slightly tweaked at time but a half cadence is still reached.
The second variation has the same basic vocal motive but the order of voices is different and the starting notes are different and there are some tweaking of chords again but it still reaches a half cadence.
The third variation is the first time all of the voices have sung at the same time. Chords are tweaked again in both function and inversion and for the first time it ends on a HC with a 7th.
The fourth variation sticks out because there is a picardy third in the resolution from the previous variation. Also notable is how many of the orchestra chords have increased at this point to four notes whereas the first few variations only had three note chords and a couple of two note chords.
The notable thing about the fifth variation ends with a minor v chord which is very strange.
The sixth variation has the picardy third again but returns to the more usual V7 chord as the half cadence.
The seventh and eighth variations both have HC with V7 chords that resolve to normal e minor chords.
The ninth variation breaks up the texture of all the voices singing at the same time and goes back to an imitative structure where a new voice is added each measure. The chordal structure also goes to the weird minor five chord as the half cadence.
The tenth variation is striking because rather than resolving to the one chord, it jumps straight to V/IV chord which is shocking in the context. The four voices continue to sing at the same time for the most part and this variation ends on the minor five chord also.
The eleventh variation resolves nicely to the e minor chord and has a normal V7 half chord at the end.
The twelfth variation is the final one. For the first time, the bass melody changes slightly to hit a good cadence. The line goes from e to d sharp to d to c sharp to c back to c sharp to d to g which signals a modulation from e minor to G major to end the song HAPPY! This variation ends with a nice PAC from a D7 chord to a G major chord.