TabControl is a container control which can not have top level control or MDI(which contain multiple forms), so what you are trying to do is not allowed by the control..
Second question's answer:-
first find your form then hide the form..
code is:-
Form childFrm = this.MdiChildren.First(frm => frm.Name.Equals("findFormName"));
if (childFrm != null)
{
childFrm.Hide();
}
I hope this will help you..