When to use "if possible" and "when possible" in a sentence?

Thanks for your question.

"if possible" means if something is possible

"when possible" means when something is possible

So the difference is when VS if.

"when" leads an adverbial clause(time) and means a specific point of time.

"if" leads a conditional clause(also an adverbial clause) and means "assumption"

For example: When possible, please call me.= when available, please call me.

If possible, please call me.= If available, please call me.

The only difference is "when" vs "if".

You should study the words "if" and "when".