I couldn’t find the score for some reason, so I’ll examine it more generally from my listening. As I listened for phrases and periods, the orchestral accompaniment was key in recognizing the structure of the piece. The orchestra’s main function was as a compliment to the voice, and merely followed the solo tenor, but at the end of many phrases and periods, the orchestra played on its own, serving either as a link to the beginning of the next phrase when the tenor comes back in or as the beginning of a new phrase (in elision). Also, in the beginning of the aria, the orchestra echoes the voice to bring out phrases through motives. The density of the orchestra also changed in the most passionate parts leading to the end of periods. Outside of this, there was little use of structural phenomena the finality of cadences. The piece was played very straight, maintaining soft dynamics and a steady rhythm and constant meter throughout. As far as measure 9-18 goes, I’ll guess which section it is. What I heard from that was an asymmetrical, contrasting, double period. It consisted of two phrases, a and a’ with very similar material and weak cadences, and then was followed by phrase b, which contained much different material and a descending pattern that gave a more finalized sound to the cadence.