The upper voice presents the theme or subject (in red). In measure 2, the lower voice imitates at the octave:
The motive inside the blue rectangle is a variation of the subject that will be used throughout the invention.
In measure five, an episode using parts of the subject (in red) starts as a dominant pedal but eventually leads to A major (dominant key):
The following image compares the original motive and the episode variation:
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