First, make two variables :
1. right (actor variable, integer)
2. canjump (actor variable, integer)
note : you can make variable in script editor by click variables>add
on Draw Actor of your Player put this code :
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- yvelocity=yvelocity+0.5;
on Collision Event with Anyside of Platform put this rule :
CollisionEvent>Anyside>Platform>PhysicalResponse>1 1 0 0
on Collision Event with Topside of Platform put this code :
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- canjump=1;
on Collision Finish of Platform put this code :
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- canjump=0;
on Create Actor put this code :
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- canjump=1;
 right=1;
on Keydown Event (to move right) put this code :
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- x=x+3;
 right=1;
on Keydown Event (to move right) put this code :
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- x=x-3;
 right=0;
on Keydown Event (to jump) put this code :
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- if(canjump==1)
 {
 if(right==1)
 {
 ChangeAnimation("EventActor","PlayerRight",FORWARD);
 }
 else
 {
 ChangeAnimation("EventActor","PlayerLeft",FORWARD);
 }
 yvelocity=yvelocity-10;
 canjump=0;
 }
 
Hope help...