what alt/ctrl shortcut often used?
Control/Alt is the default setup for shortcuts.
If you right-click on a desktop icon and click "properties" then you can enter a single character in the "shortcut key" blank and it will automatically assume that "Control + Alt +" is the first part.
A lot of people use the first letter from there: W = Word, E = Excel, E = email, G = Geekhack etc