1.there are many trees in the park.
2.there is a bird in the tree（on the tree
在树上.是指树上本来就有的东西,比如果实,花等.
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.）所以用in
3.there are five desks in the classroom