"The problem IF character JUMP and i press DOWN key he doing sit animation in air"
On the condition were you set him to do the "sit animation" add extra conditions to check that is no jumping and is not falling like this:
condition:
on down arrow presed:
player is falling: <inverted to be negative
player is jumping:<inverted to be negative
Action: set player animation to sit
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Edit: or
condition:
on down arrow presed:
player plataform is on floor:
Action: set player animation to sit
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im not sure i understand the first question but it looks like the run and sit animation the overlap when you press foward and down, do the same as above add to one of the events an extra condition like;
condition:
left arrow is down:
down arrow is down: < inverted
action: set animation to run etc....