I finally understand my problem:
My problem: I had tried using function keys to change the screen brightness -- but without success. So then I had tried changing the power settings for the computer. I set that when the computer is on battery -- it should only dim after 10 minutes. But the computer would still dim immediately when beginning battery power!
My solution: I misunderstood my function keys. I was using keys which change the brightness of lights which are under the keys of my keyboard. So now that I found the right function keys, I can adjust the brightness of the screen.
Those function keys also change the setting of screen brightness in my current power plan.
I guess the setting of dim after 10 minutes in the power plans is some other additional dimming of the screen.
THANK YOU for all who responded!
Avrohom