The Module and Class Module mechanisms in VB clarify the functional split between screens and actions. The project tree shows how the original developer organized form and function. In this case you'll have to unwind the logic back through the forms. Work with the application as a user, perhaps running test scripts, before venturing in and changing the code. Global maps and whiteboards help Global maps are especially handy when you're trying to understand large, distributed systems.
Large systems are so complex that a failure in one component can be almost impossible to locate.
Your choices are either to make no mistakes, or else be able to test components in isolation. With a map, you'll be able to see how the components of the system are coupled and how changes in one must be reflected somewhere else. And a global map is helpful beyond just the coding phase, into the planning, estimation and testing phases. It might sound like obvious advice, but be humble when approaching a complex system you didn't write. Assume nothing and question everything, especially for code that has been in production use for years.