Is there a rule for when one has to say to be a positive influence on... or to have a positive influence on...? For example,
The practice of singing songs is/has a positive influence on people.
Is there a rule for when one has to say to be a positive influence on... or to have a positive influence on...? For example,
The practice of singing songs is/has a positive influence on people.
In your example sentence, using 'is' sounds very awkward to me. 'Has' seems much more natural.
I'm not sure if there's a rule, but in general, I'd recommend using 'is/was' when talking about people, for example:
His father was a positive influence on him.
And 'has/had' when talking about things or concepts:
The presence of his father had a positive influence in his life.
One should be a good influence on others.
He had a good infuence on his peers.
In my opinion both are grammatically correct.