"看着那个站在树底下的人"怎么说?

可以是
Look at the man standing under the tree. 动名词做定语
或者是
Look at the man who is standing under the tree. 这个就是定语从句

如果用你所说的Look at he man is standing under the tree,那么到底谁是谓语呢，是is还是look？所以这个句子是不对的。

looking at that man standing under the tree
因为你用了看着
如果是看那个站在树底下的人就用
look at that man standing under the tree

Look at that man who is standing under the tree.

第一种翻译对的；

不能说；

是定语，动名词作定语修饰 The Man