this is my first time asking a question in ELL, so I am sorry if I broke any rules.
In the following sentence, is tapping a gerund or a participle?
His tapping finger on the change mat speeded up a little when Mack came in.
I thought it was a participle and that finger was the subject of the sentence, but my teacher marked it wrong. I could also see how tapping could be the subject and act as a gerund. Could anyone please enlighten me? :)