Lets suppose you have made some changes to your website and you want to redirect some pages to their new url format, this .htaccess rewrite rule might come in handy:
RewriteEngine on
RewriteCond %{REQUEST_URI} ^(.*)subpage1/subpage2/mypage.html$
RewriteCond %{QUERY_STRING} Itemid=91&page=mypage$
RewriteRule ^(.*)$ http://mynewurl.com/index.php?mypage=new
What the above condition does is redirect /subpage1/subpage2/mypage.html?Itemid=91&page=mypage url to http://mynewurl.com/index.php?mypage=new
Obviously, you need to replace the above names with your actual site names
The second RewriteCond with QUERY_STRING is only needed if the url you want to rewrite has additional parameters like mypage.html?pages=mypage for instance