The beginning of this aria (meas. 37-51) takes place after a long recitative in minor; the quick change to Eb major is a little disconcerting at first but the transition is smooth. This passage contains one very long contrasting period. The first passage begins a simple I-IV-V-I-I-V-I progression, with the voice outlining each chord and pulling us along while the accompaniment merely accents the beats. The consequent phrase has two very similar elements to begin it: first a I-vi-ii-V-I progression followed by the melody line repeating what was just said, this time under a I-V65/ii-ii-V-I progression. This phenomena of repeating the same material is used very aptly for dramatic effect, as the singer seems to be becoming more and more exaspirated, building suspense before ending her opening statement over a I64-V-I perfect authentic cadence. As for the melody, Mozart is wonderful at creating simple, beautiful and very memorable tunes. this one is no exception, the beginning very easily, outlining the one chord, then descending down and rising back up. Mozart never leaves a voice reaching for air, he always places it where he believes it to be most dramatic and effective. As the consequent phrase begins, the key word "infelice" (unkind) is the focal point, reaching the highest note (here and Ab) then descending back down, only to return again when it is repeated. Something to note about the piece, none of the cadences appears to be unauthentic, but Mozart cleverly disguises it by switching voicings, coming in with strong tonic chords after the cadences and also by repeating material for dramatic effect.