% some constants, tweak this to your liking
const GROUND_HEIGHT := 120
const RUN_SPEED := 10
const JUMP_SPEED := 30
const GRAVITY := 2

var win := Window.Open ("graphics:640;480,offscreenonly")
var chars : array char of boolean

% player position and velocity
var posx, posy : int
var velx, vely : real

posx := 20
velx := 0
posy := 400
vely := 0

loop
Input.KeyDown (chars)
if chars ('q') then
exit
end if

% to make the player move
if chars (KEY_LEFT_ARROW) then
velx := -RUN_SPEED
elsif chars (KEY_RIGHT_ARROW) then
velx := RUN_SPEED
else
velx := 0
end if

% remember, in order to jump you MUST be on the ground when pressing UP
if chars (KEY_UP_ARROW) and posy = GROUND_HEIGHT then
vely := JUMP_SPEED
end if

% subtract your "gravity" constant from the velocity EACH frame
vely -= GRAVITY
posx += round (velx)
posy += round (vely)

% simple "collision" check. just makes sure the player stays above ground
if posy < GROUND_HEIGHT then
posy := GROUND_HEIGHT
vely := 0
end if

% different colours just to illustrate whether or not you may jump
if posy = GROUND_HEIGHT then
drawfillbox (posx - 10, posy, posx + 10, posy + 20, green)
else
drawfillbox (posx - 10, posy, posx + 10, posy + 20, red)
end if

drawline (0, GROUND_HEIGHT, maxx, GROUND_HEIGHT, blue)
View.Update
delay (25)
cls
end loop

I can't get it so my guy jumps ontop of things, he just falls through.