Once you press the Alt key the menu bar will be highlighted and one letter will be with underscore i.e. the short-cut for getting sub-menu in the that menu bar. The procedure will same for sub-menu's. by pressing Alt+letter the tool will be activated.
1. suppose for closing a document from 'n' number of documents
short-cut : alt+f --->c ---- then the active window will be closed.
2. suppose for changing the orientation in the window like from isometric to trimetric view.
Short-cut : alt+t --->e ---- environment tool settings will appear on screen.