怎样把正在进行时改成情态动词句式?

你好
改为：The dog must be digging a hole with its legs
情态动词也可以和动词进行形式或完成进行形式构成谓语：表示“应当正在……”,“想必正在……”的意思.
构成：情态动词+be +动词ing 即把原来的am/is/are 还原成Be
如：
She might still be thinking about the question you raised.
　
　　At the moment she may (might) be playing with her schoolmates.
　　We needn't be standing here in the rain. We might take shelter in the hut over there.
希望帮到你

The dog can dig a hole with its legs.