The Exposition

The subject (S1) appears on the alto (A) and ends in measure 5 before the bass answers in measure 6 with a real answer (A1). A motive (we will call it a) prepares the answer in the bass. The soprano plays the subject in measure 11 and ends the exposition. Motive a is transformed by diminution in measure 7 (a2). These motives serve as counterpoints to the bass and the soprano:

Through out the fugue motives a and a2 are developed in numerous ways.

Followed by descending thirds and ascending seconds:

Or ascending thirds and descending seconds:

The seconds rhythmically displaced:

The motive seems to be related to the second subject:

And also to the third subject: