Show sub menu via hook_menu Sorry for asking such a question, but I'm still new in Drupal. I want to show sub menu via main menu.

Ex:
- Top menu
1) Home
  a. Report
  b. Comment
2) Shopping
  a. Audi
  b. Honda


In this case, I would like to see 'a. Audi, b. Honda', or go back Home and see 'a. Report, b. Comment'.
What should my module's hook_menu implementation look like ?
You can simply achieve that by using the Superfish module. 


  Superfish integrates jQuery Superfish plugin with your Drupal menus.


Image from Superfish module page



There is another module called Nice Menus 


  Nice Menus enables drop-down/right/left expandable menus. It uses only CSS for most browsers, with minimal Javascript for IE6. (Version 2 uses the Superfish jQuery plugin for all browsers, with an option to disable JS, and falls back to CSS-only for browsers that can handle it.)