There is a boy reading under the tree. 中的reading under the tree 為修飾 boy 的定語，相當于 who is reading under the tree.　　

This is Tom speaking. 中的 speaking 也是后置定語，相當于 who is speaking.　　

There is much to do 中的不定式也是定語。there is … 后接不定式與現在分詞的區別是：用不定式表示“要做……”，接現在分詞表示“正在做……”。