DELPHI中如何在登录界面，用户名和密码均正确的情况下进入到主页面

//贴个Ulogin单元,我刚完成的一个活..
//主界面Umain 在FormCreate中调用 Ulogin

procedure TFmain.FormShow(Sender: TObject);
var HLM:TAdoQuery;
sql:String;
begin
Application.CreateForm(TFlogin, Flogin);
Flogin.ShowModal;
.....
end;

-----------------------

unit Ulogin;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, DB, ADODB;

type
TFlogin = class(TForm)
GroupBox1: TGroupBox;
Button1: TButton;
Button2: TButton;
Label1: TLabel;
Label2: TLabel;
username: TEdit;
password: TEdit;
Q: TADOQuery;
procedure Button2Click(Sender: TObject);
procedure FormClose(Sender: TObject; var Action: TCloseAction);
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public de