the form uses If page = pm then show this elseif form = post message then show this....
Above is not actual code just example....
to impliment what you want you meed to make the javascript validation , the subject bar and edit the code so that it doesn't create an error ie it might be set as required....
you will need to add a piece of code that checks to see what page you are on for example ... (if sectionID = Post then) show subject box....
there would be a lot of work to impliment this..... and you would need to know how the forum works....