HTML contact form with hidden text to display on same page when submit is done

i want a basic html form with name ,email,phone number and comment fields and a submit button on the back end there is mail.php to send mail to the admin i want to write "thank you for your feedback" on the same page but it should be visible only after submit is pressed until then it should be in hidden tag similarly other texts like "enter valid email id" and "field cant be empty" i tried using but it didnt worked anyone can help me in explaining the basics of contact forms give me a basic html code and mail.php to have a good form, explain me the basic that how can i hide something in html and make it appear on any type of command or click etc. provide me a code