If you are unable to switch application-windows such as terminal windows, gedit… using Alt+Tab keys in Ubuntu Linux, then you can try fix given below:
Go to Dash Board > System Settings and select Keyboard:
Select Shortcuts tab and go to Navigation on left. Click on Switch applications setting and and set it to Alt+Tab:
Reboot the system.
If problem still persist, then you can try Compiz Config Settings Manager. To install this, type:
sudo apt-get install compizconfig-settings-manager
Open it from Ubuntu Dashboard:
Go to Window Management and check Application Switcher and Static Application Switcher:
Reboot your system, then again try Alt-Tab, it should work!!!
Tip: Compiz Config Settings Manager can help in solving many shortcut-keys related problem. For example: If show Desktop key (Alt+Control+d) is not working in your Ubuntu Linux, then you can try following steps:
Open CompizConfig Setting Manager from Ubuntu dashboard, and navigate to Key bindings tab of General options, as shown below:
Find Show Desktop setting. It may be disabled or bounded to some keys-combination.
To set it to keys-combination of your choice: click the Disable button: an Edit Show Desktop windows will pop-up:
You can, select Grab key combination, and type your keys-combination. you can use default: Control Alt d. CompizConfig Setting Manager will show it as <Primary><Alt>d
Now you can toggle to Desktop by pressing keys-combination Control Alt d