how do I display a warning before record submit in a user even script ( I still want the record to submit but display a warning first)

Wrong script type. Use a client script

^^ saveRecord entry point

and in a client script what function should I use ?? alert()??

confirm is my general recommendation

i am writing 1.0 script

implement a saveRecord event function (try not to name it saveRecord). Use something like window.confirm to confirm your user's action