A route simply connects a request method and a url to a controller method. If you have a controller method and no route, then where will the form submit to? When you specify the action, it does a reverse lookup of the route that you specified for it, then it just echoes that url/route in the html, like a regular html, form action, as HTML does not understand SignupController@Register.

How are you stuck? "I can't define a route like I did for the other form, because newPost doesn't have any url associated with it" --- this is because you haven't associated a url with it. This is what routes do. Do the exact same thing you did with your other routes. Specify a path as the first argument, and an array as the second argument with your controller method and optionally (recommended), a custom name for your route (usually the same as the URI but with dots, or slashes).

http://laravel.com/docs/routing ... see named routes