public partial class Master : Form
{
    private Form Foil;
 
    public Master()
    {
        InitializeComponent();
        Foil = new TransparantFoil(this);
        Foil.Hide();
    }
 
    private void Button_Click(object sender, EventArgs e)
    {
        this.Hide();
        Foil.Show();
    }
}
 
public partial class TransparantFoil : Form
{
    private Form _master;
 
    public TransparantFoil(Form M)
    {
        InitializeComponent();//WindowState= Maximized Opacity=0.4
        _master = M;
    }
 
    private void TransparantFoil_Click(object sender, EventArgs e)
    {
        this.Hide();
        _master.Show();
    }
}