A theme and variations is simple as far as the material discussed in chapter 3 goes. As a whole, the theme portion of the piece is obviously expository - laying out the main guideline for the rest of the piece. The first variation (and the 11 others that follow it) are then developments upon the expository.
Within the theme, the first eight bars are expository, which are the bars that include the sing-song like melody that is expanded upon later. The following 5 bars, which contain an unmarked shift in tonality, seem to be developmental because it definitely doesn't help move to a new section. The remainder of the theme returns to the melody introduced in the first period. The first variation section has the same basic outline as the theme. It's exactly the same but just arranged in a different way. The whole theme section can be clearly heard throughout it and the form is identical to that of the theme.