Here's what needs to happen to get an MDI application up and running:
1. Set the form's IsMdiContainer property to True.
2. Create a menu strip for the form.
3. Create a Window menu in the menu strip.
4. Set the menu strip's MdiWindowListItem property to the name of the Window menu.
Always refer to the topmost window with a reference to the main form's ActiveMdiChild property.
When a new child window is created, set its MdiParent property to the main form.