I've recently been having trouble with my Macintosh and I spent a couple of hours today online with Apple support. During the phone call they asked me to do several things when restarting my computer, things like chicken diagnostics, safe booting, Internet recovery, and reinstalling the operating system. A lot of these are covered by keyboard shortcuts you restart your computer.
Diagnostics – hold down the D key when restarting the computer before the chime. Keep holding the key down until you see the progress bar.
Safe Boot – hold down the Shift key when restarting the computer but after the chime. Keep holding the key down until you see a progress bar.
Internet Recovery – hold down the Option+D when restarting the computer but before the chime.
Recovery – hold down the Option key when restarting the computer before the chime, this will allow you to choose whether to restart normally, or to access the recovery drive.
Resetting PRAM (NVRAM)– hold down the Option+ Command+ P+ R keys before the chime, and keep holding the keys down until you hear the child for the second time.
Here are some other startup key combinations: