on our keyboard there are 3 special keys which either revee\rse the normal command or give some special command. you must have guessed them easily as SHIFT. CTRL and ALT keys.

although the best way to find their use is to try them on own , here are some of them .
Use of Shift , ctrl and alt keys.

1. You can use the Shift key in AutoCAD in many ways to help make your AutoCAD tasks easier and quicker:
2. Press Shift with the FILLET or CHAMFER command to change the radius to 0 and extend to lines to meet
3. Press and hold Shift to temporarily override ORTHO.
4. Press and hold Shift+A to temporarily override OSNAP.
5. Press Shift key and click on selected objects with mouse, to deselect.

Ctrl key :

1. Press Ctrl + A to select all objects.
2. Press Ctrl + 0 ( zero ) to remove all toolbars and menus. This is useful for expert users who know the commands by heart and use keyboard shortcuts.

we will go on adding to this list and you are also requested to add any of such ideas , which can be shared.