Problem with setting an onsubmit event

Problem with setting an onsubmit event

I have a page with several different forms. One of these currently has an onsubmit event to run a custom validation program before the action is triggered. This event is currently shown as
  1. onsubmit="return validateForm(this)" 
and works as expected.

I want to be able to use this form in some instances without this validation so I want to take this event out and add it if a certain field has been set before the page was loaded. I'm new to jQuery and thought I'd understood how I could do this but I'm not getting this to work.

First of all I added a class of 'mainform' to this form and then used the following to set the event:
  1. $(document).ready(function() {
  2. $('.mainform').onsubmit = function (event) {
  3. return validateForm(this);
  4. };
  5. })
I've tried putting line 4 in quotes but it still doesn't work. What am I doing wrong?

Thanks
Eric.