Mother dragged a screaming Phillip to bed.

How do you explain (to an TEFL student) the use of a (article) in this case.

I can`t find a rule.

Is screaming a gerund or a verbal noun? It`s functioning as an adjective!

Thanks, Solatedo.
Yes, it is an adjective-- and being so, can divide Philip up (via secondary reclassification) into several different personalities, several different Philips, who thereby become countable.