how can i get if a player on ground or not?

Subtract 0.1 from the location then see if the block of that is air.

 

whats wrong with https://hub.spigotmc.org/javadocs/spigot/org/bukkit/entity/Entity.html#isOnGround()?

 

It returns if the player sends onground packet. If you use a hacked client u can just send that your onground and this will be true event if you arent onground serversided

 

Code (Java):

if (!player.isFlying() && player.getLocation().subtract(0, 0.1, 0).getBlock().getType().isSolid() /* is that a method ? */) {
    // on ground
}