You could use properties, or parameters in the constructor. eg.

-- loginForm.cs --
MainForm frm = new MainForm(txtUser.Text, txtPass.Text);
frm.Show()
-- / --

-- MainForm.cs --
public MainForm(string user, string pass)
{
    // Put user & pass into private fields
}
-- / --

Or alternatively, add some properties, so you can do

-- loginForm.cs --
MainForm frm = new MainForm();
frm.User = txtUser.Text;
frm.Pass = txtPass.Text
frm.Show()
-- / --

Though if you're doing it that way, a struct to hold the details might
be better:

-- loginForm.cs --
MainForm frm = new MainForm();
frm.UserDetails = new UserDetails(txtUser.Text, txtPass.Text);
frm.Show()

struct UserDetails
{
    public string User, Pass;
    public UserDetails (string user, string pass)
    {
        User = user;
        Pass = pass;
    }
}
-- / --

HTH