What's the object name of the initial form in a windows form application?

Question

Hey

After some time, I'm started again into the language C#. I have a windows form application with three different forms and I have functions that are interacting with more then one form. An easy example is the close-Button. It should not only close the currently active form but also the main form which is invisible in the background. Also the "Back to MAin Form" button has the same problem, because I don't know how the instance of the main form is called. How can I find out how the instance is named?