可以帮我逐句分析下下面的代码吗？我想做一个登录系统，非常感谢！！！非常感谢！！！
Private Sub Command1_Click()
Dim mrc As ADODB.Recordset
txtsql = "select username from use where username='" & Trim(Text1.Text) & "'"
Set mrc = ExecuteSQL(txtsql)
If mrc.EOF = True Then
MsgBox " 用户名错误！", vbExclamation + vbOKOnly, "警告"
Text1.SetFocus
Text1.SelStart = 0
Text1.SelLength = Len(Text1.Text)
Exit Sub
End If
username = mrc.Fields(0)
txtsql = "select username from use where password='" & Trim(Text2.Text) & "'"
Set mrc = ExecuteSQL(txtsql)
If mrc.EOF = True Then
MsgBox " 密码错误！", vbExclamation + vbOKOnly, "警告"
Text2.SetFocus
Text2.SelStart = 0
Text2.SelLength = Len(Text2.Text)
Exit Sub
End If
MDIForm1.Show
Unload Me
End Sub