harmony progressions

[quaver]

I have just read one of Bernhard's comments and he said if you need to know anything, just ask questions, ask question, ask questions.  Here goes.  I do hope someone replies.  I really need assistance. 

I am studying JCF BACH'S Allegro in G Major.  In order to memorize it successfully I am trying to figure out the harmony progressions.   This piece is in rounded binary form.  The B section (starting at measure 17) is where I am perplexed.  I believe it is the dominant of a minor leading to the tonic of a minor in measure 18 and then still in a minor for measures 19 and 20.  The G# is now missing in measure 21 so I suppose the key is no longer a minor. So now I am stuck from there.
Hope someone can help me out with these chord progressions.

[mad_max2024]

Can you post an image of it?

[quaver]

Thankyou so much for replying.  I do not know how to post an image but I will certainly give it a try.  Hold on, it may take a while!

[quaver]

Wow, I don't know how I did that so fast.  There it is anyway.

[mad_max2024]

I'll give it a shot.

It seems to start in A minor.
First beat starts with the E major chord which is the dominant then resolves to A minor.
The G sharp comes from the harmonic scale and the E major dominant, the F sharp from the melodic scale that he uses to fiddle around.
Then he goes back to G major, maybe in bars 21 and 22.

Did that make sense? 

[nystul]

The first clue that things are changing should be the F# in the bass at m 21.  Essentially you've gone from Am to a D7 which leads into a series of D7 -> G resolutions.  Notice also in m 23 on the third count with the C#, I would think of that count as a secondary dominant A7, leading to D (D7) to G.

[quaver]

Would you think that measure 20 has two key changes.  Beat 1 is e minor to accommodate the d# and beat 2 in a minor to accommodate the g#.  So measure 21 is V7 of G major.   Do I take it that the B section starts in the a minor but by measure 21 reverts back to the home key of G major. (should I be seeing a pivot chord somewhere). A minor to G major seems a strange progression. So measure 23 is the secondary dominant chord of G major i.e. A major and measure 24 is the V chord i.e. D major.  The key of a minor is short-lived methinks.