Question

There is an apple * the tree,*填什么：on 还是in

Answer 1

用 on the tree!解释如下：in the tree表示外部的东西落在树上,例如风筝落在了树上啊,或者树上的一只鸟等,应该用介词in. eg. The birds are singing in the tree.
on the tree表示树上本身自有的东西.例如树叶、果实长在树上等,应该用介词on.
eg. There are a lot of apples on the tree when autum comes.