Display Main menu only on login page
You have 3 simple options (And even more with preprocessors, but I won't go into that):


Most themes auto output the primary and secondary menu (configured in Appearance -> Global/THEMENAME -> Settings). You can turn it off on the theme settings, and use the Block system (Structure -> Blocks -> Main menu -> Configure) to put it on the pages (2 valid urls):

/user
/user/login


pages only.
(This is exactly the same as 1 except it's not database configuration, it's a code edit) Edit the page.tpl.php and comment out the lines that go something like:

if($primary_menu) { print $primary_menu; }


and again use the block system to output the main menu block on the login page (2 valid urls - see 1)
Or alternatively you can create a new custom menu, stick your form link there, and use the block system to assign it to the login page (2 valid urls - see 1). And leave your Main menu empty.