Here are some helpful keyboard combinations that are helpful for Mac troubleshooting (The command key is the key with the cloverleaf symbol). The following keyboard combinations should be performed immediately after Mac power on or Restart ...
Force the Mac to boot from OS X, no matter which disk is specified as the startup disk:
Hold the 'x' key during startup.
Boot from a bootable CD or DVD:
Hold the 'c' key during startup.
Boot from a networked computer that has a NetBoot volume:
Hold the 'n' key during startup.
Use any Mac with a FireWire port as the source for your bootup system:
Hold the 't' key during startup to boot in FireWire Target Disk Mode (May not work in latest models).
Begin Apple Hardware Test with Apple Hardware Test DVD in the optical drive:
Hold the 'd' key during startup.
Get OS X startup manager to appear to select a boot disk:
Hold the option key during startup.
Boot your computer in Safe Mode, which disables login items and non-essential kernel extensions:
Hold the shift key during startup.
Boot your Mac in Verbose Mode to use descriptive text sent to the display during the startup process:
Hold Command + 'v' during startup
Boot your Mac in Single-User Mode -- a mode used for troubleshooting and repairing complex hard drive issues:
Hold Command + 's' during startup.
Eject a CD or DVD from the optical drive during startup:
Hold down the mouse's primary key during startup. On a multi-button mouse, the primary key is usually the left button.
To zapping the PRAM -- return the Mac to its default configuration for display and video settings, time and date settings, speaker volume, and DVD region settings:
Hold Command + Option + 'p' + 'r' during startup. Press and hold the key combination until you hear the second set of chimes.