on,in
on the tree在树上..是指树上本来就有的东西,比如果实,花等..
举例：The apples are on the tree.
苹果在树上.
in the tree在树上..是指本身不属于树的东西藏在树里面了..比如人或动物爬到了树上..
举例：The cat is in the tree.
猫在树上.