In case anyone was wondering, here is the smallest loop structure that I know of in Axe.

By smallest, do you mean in terms of size, or in terms of speed?  (or both?)

You could try flashbook:
http://www.ticalc.org/archives/files/fileinfo/418/41870.html

I haven't personally used it before, but I think it fits all of your requirements.

Ah...
I finally understand this:
0->X
While +26
  Do stuff...
  X-2->X
End

This is incredibly helpful - thanks!

This has probably been already asked before, but how about a 'break' function?

Also, it would be nice if I could do something like 'BreakIf A' similar to the already existing 'ReturnIf A'

I might be completely wrong, but I think part of your problem was that in your pxl-Tests (in your subroutine, PLT): every time in the loop the pxl-Test was false, you added (or subtracted) the velocity from your y-coordinates.  This meant that your sprite would go much futher then you wanted.

(Also, I'm assuming pgrmALEV drew some kind of floor or something, so I replaced it with a line command) to demonstrate.

I added your code (optimized):
.HOGGY

[007E425A5A427E00]→Pic1

1→L→F→G→V+4→Y*5→X

ClrDraw
Line(1,60,90,60)
Pt-Off(X,Y,Pic1)

.Main loop
While 1
Text(1,1,G►Dec)
Pt-Change(X,Y,Pic1)

If getKey(15)
ClrDraw
Return
End

.Control movement
getKey(3)-getKey(2)+X→X

.Gravity Acceleration (this doesn't need to be in a subroutine)
If G=1
V+1→V
End

.Collision test
Sub(PLT)

Pt-On(X,Y,Pic1)
DispGraph
 
End

.Subroutines begin here
Lbl PLT
If V
For(I,1,V)
If pxl-Test(X+1,Y+I+5) or pxl-Test(X+6,Y+I+5)
Y+I-2→Y
0→V
0→G
Return
.If something is hit, the program returns to the main loop
End
End
.If I went through the For loop without hitting anything...
If F         .(I'm assuming that F can only be 1 or 0 here)
Y+V→Y
Else
Y-V→Y
End
End
Return



Awesome game, although I found the key response to be a tad sluggish, especially when turning the pieces.

Just a question - in the readme, you refer to the game as 'Eittrix', but the title for this thread is 'Eitrix'.  Was this intentional?

And also, is there a more appropriate place to put this topic? I know that there is a Calc Projects board somewhere, and I think that that board would be a better place to move this. What do you think?

I agree, mov'd

In short:
Labels jump to a certain part of your code.  It's useful for things like menus -- use a goto, and imstamtly jump to the start of your progran (if that's what you want to do).
Subroutines are useful for reusimg code.  For example, let's say that every time I press [2ND], I want to do somethimg complicated, then create a bullet.  Instead of typing out the code that'll create a bullet every time I use the [2ND] key, I can just use a subroutine (which jumps to the subroutine, runs the code, then jumps back where you were) so that I have to type out the code only once.  In addition to being more convenient, subroutines also save memory compared to writimg it out each time.

Before today, I wasn't too sure if I would continue this project because I couldn't get the shadow effect to run at a fast enough speed to merit making any kind of game based on the effect.
However, with the faster line commands plus drawing lines to backbuffer, I could actually increase the speed even more (I hope)
Using the shadow effect is definitely feasible now, so I'll be resuming this project.