Try these steps
- Login to admin panel and navigate to CMS -> Pages.
- Create new page.
- Add URL Key as main-url/sub-url for example if your main page url is aboutus then sub page will look like aboutus/aboutus-sub
- To add those links to left sidebar, you should create phtml file in your theme template folder and call that phtml file from appropriate xml file.
- <reference name="left">
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