chodzi ci o wymowe?
Hmmm, I remember when I was teaching my youngest to read he had difficulty differentiating between short "a" and "u".
This is what I tried with him:
short "a" : cap, bank, bat, ankle
pronounce each word with an exagerated "smile" (the shape of the mouth moves horizontally)
short "u": cup, bunk, but, uncle"
drop the jaw in a relaxed posture, in the vertical direction.
Let me know if it helps