I have no idea where to begin with this Fugue. I have missed the lectures both days and didn't get into the library in time to hear it. Anyhow, here goes.
The main theme is exposed in the left hand in mm. 1-4. The right hand then comes in with the theme, though it is modified, transposed to tonic instead of starting on the IV chord. The third voice enters with the theme in its original form on beat 3 of measure 9.
I would guess that the modulatory section begins in measure 31. Leading up to that, the last three measures follow the cadence V-I-V7. In measure 31, the left hand plays a pattern which is repeated again in bar 33 a step lower, leading to a series of tonal centers that are not tonic. The piece wanders from I to vi to V/ii to ii and then stays in V/ii for a while (measures 36-41). It seems the piece has modulated to ii at this point and remains there until m. 68, where the terminative section begins.
The melody is reexposed in measure 41 in the left hand, as well as m. 47 and then 48 in the middle voice.
Measure 68 begins the terminative section, which just reinforces the tonal center of F. I bet it sounds relieving b/c the piece went through so many keys to get there. Also, the cadence leading up to the return of f was very extended. Beginning in measure 61, it goes:
V7/IV, IV, V65, V7/IV, II6, V7 (though I6 is hinted at in m. 66), V43, and finally I.