Purcell's aria "Ah, Belinda, I am prest" from Dido and Aeneas is an overall simple binary (AB) form but uses continuous variation, particularly passacaglia, to connect all the sections together. The bass line to this aria is very important because it is the tool used to connect the sections while the upper parts provide times of variation. The bass line repeats throughout the whole aria with the variation happening either in the voice line or upper part of the continuo. Purcell is very clear when he modulates into g minor in measures 45-52 because the bass line suddenly switches to the same note pattern, only using pitches from g minor scale. The A section starts with the vocal line slightly imitating the passacaglia bass in measure 3. To me, this imitation on "ah!" says that Purcell's passacaglia was meant to be the continuous sigh throughout the aria. To contrast with the simple bass part, Purcell makes the vocal line very elegant, rhythmically interesting, and easy to embellish. The singer should probably emphasize qualities in the vocal line, such as the sigh, by leaning on the chromaticism, embellishing dotted rhythms, and inflect the text that Purcell paints so well.
In the repeated A section starting in measure 18, Purcell uses a rhythmic crescendo to create more tension and draw more interest from the listener. Since the basso continuo part does have that moving line, as a singer I would try to make this repeat more legato than the first and worry about text more in the first take. This will give a chance for the basso continuo to be heard better and for the singer to give a different color to their tone. I think Purcell would want the singer to ornament the heck out of measures 25-33 since there's no variation in the bass.
In the B section, starting in measure 36, Purcell puts a direct imitation of the passacaglia in the voice. I interpret this move as Purcell's way of switching from "voices." In section A, Dido is clearly speaking to Belinda- an objective, extroverted statement. Perhaps this B section is an inner soliloquy; a more personal side of Dido. Once again, the initial statement I think, should emphasize the text. The singer has to wonder why Purcell chose to place the move from tonic-leading tone on the word "peace." Perhaps he's foreshadowing the tragic end of the opera by showing that Dido is already thinking of a more permanent, morbid peace. The text is also important because of the text painting Purcell sets on "strangers." It's important for the audience to hear the moving line. I think the singer should also bring out the difference between the viio42 harmony on "grown" in measure 39 and the i in measure 44. Is Purcell implying that Dido has become more comfortable in her relationship with peace?
So my question is, why did Purcell decide to modulate in measure 45. I'm guessing it has something to do with a change of mindset. Just when Dido becomes comfortable with her situation, she realizes that she's actually screwed- a great reason for a change of key. This also gives Purcell the chance to put the theme in a range allowing for a more interesting color for a mezzo. Especially on the word "languish;" he must really want this word to win the prize. It lands right on the modulation, takes up two measures, and spends an entire half note on the leading tone. The second "languish" spends most of its time on dominant, moving eveuntually to a viio in measure 52. The third "languish" in measure 53, starts on a neopolitan 6 chord and the vocal line and basso continuo part move together, which I interpret as Purcell's way of getting an even darker color than already portrayed- he must have really wanted "languish" to be the cheese of the song.
The little interjection in measures 56-63, I think are just Purcell's way of getting back to c minor and having some declamatory statements before the return of the inner soliloquy. Here in measure 56, we see the same i, viio juxtaposition, only this time it's within one measure. The suspension in measures 56-57, dotted rhythms in measures 58 and 59, and overall thicker texture, demonstrate a nice contrast to the coming measures 60-61. Like the beginning of the piece, the first part of this section (56-59) should be more declamatory and text driven, and the second part (60-63) should be legato and more personal.
The return of the B section in measure 64 should be performed in a more helpless tone, with an "unfinished" presentation. Even in measure 67, there are more eighth notes in the basso continuo, a sign that the while the voice may end, the instrumental part will finish Dido's thoughts for her, since it mirrors her subconscious. The instrumental part does move on in measure 72 by repeating the passacaglia, which will be continued until measure 84. I think this section should be the basso continuo's chance to ornament and express more freely since they don't have to worry about the singer anymore. The singer should probably present themselves as a kind of "living dead" during this instrumental closing. I think Purcell is implying that Dido's feelings are incomplete and in a way, inarticulate because of the lack of the vocal return to the A section. Dido just suddenly stops talking to Belinda, and even herself, and the basso continuo has to finish for her because she's so emotionally unstable. Overall, I think one of the singer's main goals in performing Purcell is to make sure the text is well-communicated. I think he's a genius at setting the English language, something few composers do well and usually the text plays an important role in his songs. I also think that his use of the text, mood changes, and drama is ahead of his time, so the singer shouldn't be afraid to really bring this out in their performance of any Purcell song or aria.