The following key combinations can be used during power up to access various repair/troubleshooting modes on a Mac. Unless otherwise stated all hotkeys should be pressed as soon as the startup chime sounds after pressing the power button and should be held until the apple shows on screen.
Single User Mode
Cmd(?)+S
Repair Mode
Cmd(?)+R
Reset PRAM/NVRAM
Cmd(?)+Option(?)+P+R
Diagnostics Mode
D
* hold until language selection displays
Internet Diagnostics Mode
Option(?)+D
* hold until language selection displays
Internet Recovery
Cmd(?)+Option(?)+R
Startup Manager
Option(?)
