Qual è la differenza tra demonstrate e substantiate ?Sentiti libero di rispondere con un esempio.

They can be used, in different ways, to mean the same thing but are not interchangeable words.
Let's say I claim something and then someone says "That's not true." you could then respond:
"I can demonstrate that it (that claim) is true" or
"I can substantiate it (that claim)"
So, effectively, substantiate means "to demonstrate something is true" (to "back something up with proof").
It's possible to use them in the same way, however, when it's implicit that you're trying to prove something to be true. If you were in the same situation as before you could respond:
"Yes, it is. I can demonstrate it"
"Yes, it is. I can substantiate it"
The "Yes, it is" explicits that you believe it's true, so "demonstrate it is true" can be shortened to just "demonstrate it"

In short, "to substantiate it" = "to demonstrate it to be true"