This shows the first two bars of Schumann posthumous variations V.
Look at this music note (red-squared), I am curious to know why Schumann chose a natural G instead of the original flat G in this original D-flat major (or B-flat minor) piece?
It sounds a bit abrupt to me that Schumann chose a natural G for this music note (red-squared). So I hope that there is a wise explanation/interpretation? (Please feel free to give me some comments/pieces of advice/background educations on this piece.)
The G natural is the chromatic lower neighbor to the A♭. It parallels the A natural that occurs in the preceding beat 2 as the chromatic lower neighbor to the B♭.
This use of chromatic lower neighbors is casually motivic. The below reduction shows (in red) where they are used is measures 1–8. In the specific case asked about, the underlying chord is D♭ major. Using a G natural intensifies the focus on the A♭, which is the primary melodic pitch on beats 3 and 4. The B♭ on beat 3 is a suspension from the previous measure (or, if preferred, an accented upper neighbor).
I have a low-level observation on the following two melodies:
The red part sounds a bit abrupt that was the question.
But the blue part middle-line melody is totally reasonable. There are two parallel phrases in the second bar as the middle-line melody. The middle-line melody may be a good reason for choosing that natural A note in the first phrase, while choosing that natural G note in the second phrase. (This is what Aaron says I think.)
But the red part is still a bit strange or mysterious to me.