start_form is used when you have multiple scripts (forms) to be printed in one output. It can be called many times and closed by close_form.
open_form opens the form for printing (called only once) and should be closed with end_form.
For more information, please check this link.
Open_form => It assign the form and printer, It should be first.
Start_form => It start Writing mode. You can use write_form in loop to write more than one lines befor End_form.
End_form => It end writing mode of current page and will require to start again through Start_form.
Close_form=> it end the Form. After this you can not start again for created file.
reward all helpful replies.