1)which should be used(recommended)?
2)why?
3)If glsl really does gives you more control on graphic rendering,why is the tradisional opengl still used?
4)why is glsl so hard to learn,can't the creator make it easier?
Well, glsl goes back to 2004. If you had to do any graphics work prior to that, it was very painful and much more complicated.
Most was done in a hardware specific language or in assembly language prior to that.
Unfortunately even the best of the old is just that, old and more complicated.
Things always get improved as time goes on, today there's Qt which is quite easy to use. tkinter, the built in GUI is
pretty easy to use (except for the geometry commands), you can still create fairly nice apps using it
wxpython, though not as popular is also a good one.
Bottom line, GLSL if taken in perspective is pretty good.
It depends on what you're doing. If you're writing shaders, glsl is pretty much your only option.
Realistically, you're going to be using opengl, with a little glsl splashed in.
Think of it like html+javascript. You can write just html fine, but a little javascript makes it "better". Just javascript by itself is useless, though.