Hello,
Preposition "about" and "on" are exchangeable ?

Chris is speaking on the abc Project.
Chris is speaking about the abc Project

This is the story about elephants.
This is the story on elephants.

What is the general rule ?

I am sure these are not "interchangable". I know it sounds very obvious, but they are only interchangable when they have the same meaning. And, they don't always have the same meaning.

Is that a finger print about the window?
It's your mum about the telephone.

So, they are clearly not interchangable. When on means "about", as in your examples, they are largely interchangable. For me though, "on" has a different connotation completely.

He is talking about elephants.
He is talking on elephants. (I envisage a lecture on elephants with this one.)

Can you see the difference?

Rgds

Craig

This does not make sence.If you are tring to write a poem be more creative.Repeating the same sentence over and over on the actaap will get you below basic.

A preposition tells you exactly where a thing is. For example: The bench was beside the ice cream van. The preposition would be beside.