All the Special Key Combinations that Change Your Mac’s Startup

• Shift: Starts your Mac in safe mode. This helps you troubleshoot because it only loads the minimum necessary kernel extensions at boot then disables startup items, user-installed fonts, font caches, kernel caches, and other system cache files. Safe mode also runs a file system check automatically, which should help with troubleshooting.
• Option: This loads up the startup manager where you can pick between different hard drives or discs to boot into. If you need to boot from a hard drive different than your primary one, or you’re booting into Boot Camp, this is the key you push.
• C: Boots from a bootable CD, DVD, or USB. This is useful when you’re installing a new operating system.
• D or Option+D: Starts the Apple Hardware Test on pre-2013 Macs or Apple Diagnostics on newer Macs. Both are meant to help troubleshoot hardware issues.
• N or Option+N: Starts up from a Netboot server. Most average users will never need to use this as it’s meant for running OS X off a network instead of a hard drive or disc drive.
• Command+R: Starts up in Recovery mode. If you have problems with your hard drive, OS X Recovery allows you to restore your Mac from a backup, verify and repair your disc, check your internet connection, or reinstall OS X.
• Command+Option+R: Starts up the internet version of Recovery mode, which works the same as regular Recovery mode, but is online.
• Command+Option+P+R: This resets the NVRAM. NVRAM stores information about speaker volume, screen resolution, startup disk selection, and recent kernel panic information. If you’re having issues with sound or video, it’s usually a good idea to reset the NVRAM before panicking.
• Command+S: Starts up in single-user mode. This is meant mostly for developers and IT as a means to troubleshoot startup issues and basically drops you into the command line where you can run tests without worrying about the GUI in OS X.
• Command+V: Starts up in verbose mode. Verbose mode is similar to single-user mode but is meant more as a way to watch what a computer is doing to help with troubleshooting.
• T: Starts your Mac in target disk mode. This is a useful way to share files between two Macs when one of them is broken or the display isn’t working.
• Eject button, F12, mouse button, or trackpad button: Force eject an optical disk.

Source: All the Special Key Combinations that Change Your Mac’s Startup