The cat in the tree is small.在树上，不是生在树上的，要用in。
Please give this new pencil-box to them.介词to表方向概念。
That's a jacket.It's a brown jacket.缺少冠词。
Is everyone here now? -No,Li Lei isn't here. everyone在此是单数概念。
The man behind the tree is my mother's brother.专门指树后的那个人，对象是特定的，应该用冠词the表示特指。