Hi Amuait.
If you are referring to an MDI application then:
1. Set the property "IsMdiContainer" to "true" in your main form (Parent Form).
2. For each form you wish to open / show within the parent form:
AForm MyChildForm = new AForm();
AForm.MdiParent = MyMainForm;
AForm.Show();
If the above is not what you're looking for, could you explain your requirement in a bit more detail....