Its just my opinion, if you want to take my feedback in to consideration then do that, if not just ignore it.
The physics are bad because it only has 3 different functions:
1: move up if you press up.
2: stand still if you land on platform
3: fall down if 2 is false.
This is good in itself but you could try to add some more realism to it by doing something like this:
global inAir
global ySpeed#
global y#
do
if inAir = 0
if upkey()
ySpeed# = -2.0
inAir = 1
y# = y#-1.0
endif
endif
if "collide with platform"
inAir = 0
y# = "platform height"
ySpeed# = 0.0
endif
if inAir = 1
y# = y# + ySpeed#
endif
loop
9cdfb439c7876e703e307864c9167a15