Modal vs. Separate Page: UX Decision Tree

You probably have been there before. How do we choose between showing a modal to users, and when do we navigate them to a separate, new page? And does it matter at all?
Actually, it does. The decision influences users’ flow, their context, their ability to look up details, and with it error frequency and task completion. Both options can be disruptive and frustrating — at the wrong time, and at the wrong place.
So we’d better get it right. Well, let’s see how to do just t