In this article, I would tell you about Drupal menu functions:

In this article you will find a list of main Drupal menu constants with descriptions and code examples.

In this article I will teach you how menu works in Drupal and how you can use it & modify it.

In this article, you will learn how to create menu structure on-the-fly, adding new content to your Drupal-based site.

In this article I would tell you about:

Dividing main and secondary menu items
Secondary items show when one of the main item is selected
Highlighting of parent items at every level with an active class
Ability to show …

 
You need:

Create split-level menu.
The upper string is main items (first level), and sub-menu appears to the left of the selected item.
Parent item from main menu items is active when navigation through submenus.