When to call on the river?
Basically, it will depend on how the hand was played up until that street, but in short, a call is profitable on the river when you have a
bluff-catcher, that is, the villain has a lower value than you and must have a bluff frequency, because for the river call to be profitable, you must win more than 50% of the time.
I know it can be a bit complicated to understand just in theory, I'm also in this process, however, you can give us an example of a hand that you played and we can analyze it together.