I still can't understand what a difference between a regular window and modul form ? what it means modal form?

In regular windows you can switch between parent and child windows, on the other hand you can't.

Dialog "forms" are either modal or modeless. Modal dialogs are the most common. A modal dialog changes the mode of input from the main application Form to the dialog. When you display a modal dialog you cannot switch between the dialog and another form of your program. Example: a Save dialog is modal, you either have to save a file or cancel the save, you can't do anything else in this program. Modeless dialogs are like a Find dialog. They stay on top but you can still do something in your main Form.