Patterns
Okay so this is a pretty banal theme, with some double counterpoint so the alternate notes can be played both above and below the initial line, which is then done. The theme, such as it is, is varied in various ways, notably inversion, retrograde (a fancy way to say "play it backwards") and augmentation (near the end, with free high notes to distract one from how bad the theme is). Augmentation is a fancy way to say "play it slower". The inversion probably needs more detail, as there are various ways to invert notes; see "Fugue and Invention in Theory and Practice" by John W. Verrall for more details. Lacking that text, some inversion charts should get you started:
c d e f g a b c' Original 1 2 3 4 5 6 7 8 Inversion 5 4 3 2 1 7 6 5 g f e d c b a g Original 1 2 3 4 5 6 7 8 Inversion 3 2 1 7 6 5 4 3 Original 1 2 3 4 5 6 Inversion 6 5 4 3 2 1 Original 5 6 7 1 2 3 4 5 Inversion 5 4 3 2 1 7 6 5
The last is for minor mode, though the above piece is in minor but does not use the last one. Chromatics raised in the original should be lowered in the inversion, and probably many other details too long to repeat here.
Several other patterns are possible that were not used in this work, in particular retrograde inversion (turn the theme upside down and play it backwards) and diminution or "play it faster".
Source