The A section is in the key of g minor starting in measure 4, due to the amount of leading tones at the beginning, but it is kind of difficult to say because of the occurence of Ab's later on. Actually, the more I look at the score, it's very hard to tell what key this is in. I thought of the Ab major chords as Neopolitan chords. Anyway, The left hand begins the piece the first four measures with a very heavy feeling; the mood of the piece changes many times. I felt an IAC at the end of measure 12, but only very briefly. I felt section A was a period made up of 2- eight measure phrases. Measure 21 repeats the motive in measure 19, and Bartok plays around with it rhythmically through syncopation, giving it more of a compound as well as an accelerated (yay) feeling. I felt this section (measures 21-27) as being terminative as well as a transition to the B section.
Measure 28 begins the B section, this time with many added Db's, as well as some Gb's. There are 8 measures of kind of a call and response between the left and right hand, until in measure 35 they being to mesh together more. I felt like maybe this could be in f minor, because the f seemed the most tonic to me. Again, Bartok majorly plays around with syncopation, more in the B section than anywhere else. Measure 46 is terminativeand again, transitioning again back to the A section. It ends tonally open.
The return to the A section isn't exactly like the beginning A section, and is more of a variation of the A section. You can still hear the melodic line standing out from the 16th note runs, however. The A section ends tonally closed (because the piece itself wouldnt be closed) The tempo by this time has changed a little, but sounds much faster because of the 16th notes. I noticed that each section in this example of ternary form is independent from each other, and is additive.
The left hand also just plays the chords throughout the piece, with the melody being solely in the right hand.