Above is the opening subject and counter subject.
My two questions are about the transformed counter subject in bar 5. When restating the counter subject why does Bach:
- Displace the circled D by an octave?
- Extend the following C with a dot?
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There are two especially important implications of Bach's choices here:
In addition, the D (1) gives a sense of a cadence, with the remainder of the countersubject sounding interrupted. This is another reason to prolong the C – it draws attention back to the countersubject.
For a more precise explanation of the A-D descending fifth in m. 5, see Michael Curtis's answer below.
When the motif is sequenced in mm. 3-4 the joinings result in a descent of a fifth between beats...
...one way to explain the countersubject descending a fifth instead of ascending a fourth in m. 5 is it reflects the descending fifths of the preceeding sequential line.
For the dotted rhythm you can look at it like this...
...the main motif starts with an anacrusis rhythm. The dotted rhythm and sixteenth just extend the line back. You can think of the countersubject rhythm as the down beat eighth notes and the main subject as sixteenths with an anacrusis.