Hold these keys when Mac is restarting, to enter various modes. These are for Intel Macs
Option - to Startup Manager (select disk to boot).
Shift - to Safe Mode (forces your computer to load only the bare essentials it needs to start the operating system. Your startup disk will check for errors and make repairs, and only use the required kernel extensions. This mode also prevents the startup and login items from opening when your Mac starts up. It disables custom user-installed fonts and clears a number of macOS system caches too.).
D - diagnostics (hardware test).
Cmd+Option+P+R - reset NVRAM (previously known as Parameter RAM).
Shift+Ctrl+Option - reset SMC settings (System Management Controller, chip that controls Fans and Power management).
Cmd+R - boot to recovery partition.
Cmd+Opt+R - boot to internet recovery mode.
T - target disk mode (Mac turns into external HDD for another connected Mac).
Cmd+S - to Single User Mode (minimal text-based environment with no GUI and no drive volumes mounted on startup).
Cmd+V - Verbose mode (logging to console, You can see exactly what interrupts the boot process).
C - boot from CD/DVD.
F12 - eject CD.
Na - boot from network image.
Opt+N - boot from default network image.
Left Shift (after Apple Logo appears) - disable automatic login and go to login prompt.
Shift (after entering macOS password) - disable login items and disable restoring previous session for the current boot.