除非选中复选框，否则禁用表单提交

I need to tweak the code for a basic form. I'm not a PHP developer, I'm a graphic designer and I have just basic knowledge of PHP. What I need to do is to disallow this form from sending unless the checkbox is ticked. I found some code on the web, that I think should do the trick, but it needs some changes and I'm not sure which part of code to tweak. I looked into the PHP and I'm aware that there is a good bunch of code that I don't need, I see there is also a JavaScript, I already changed the form layout in HTML and left just what's needed. I need to tweak the code for a basic form. I'm