How to determine where control is.

I hate to ask such basic questions, but how do you know when you've moved to the next page. Where will you be running, or rather, what event will fire, and how do you know which page you're on at that time? I know these are basic questions, but I feel like I am wasting time unnecessarily trying to find the answers. You'd think there would be a basic manual where you could look these things up.
Thanks,
Mark