This sonatina is pretty much straight-forward, and I believe everyone has played it once (if you didn't you should. It's a very famous piece and a good exercise for the left hand-arpeggiated notes). The piece is mostly I-V-I... , but then it modulates to the dominant key (V),(G DUR).
There are several vii half diminished chords which are usually followed by I.
The B section begins with V7 chord followed by i MINOR which is obviously mixture chord. Those 8 measures (from m.16-m.23) are great example of modal mixture.
I MAJOR/DURRRrrrrr.... The left hand brings the first theme again just one octave lower.