• Safe mode : Shift after startup tone and until login screen
• Other boot disk (USB etc.) : Option/Alt (⌥ on Macs, Alt on Windows) after startup tone and until startup choice appears
• Verbose : Command (⌘ or Win) + V
• Hardware test : D immediately, until test windows
• Reset PRAM/NVRAM : Command (⌘)—Option (⌥)—P—R, then release the keys after you hear the startup sound for a second time
• Reset SMC: press and hold the left-side Shift (⇧)—Control (⌃)—Option keys along with the Power (⎋) button, all at the same time, until the computer turns on. Now release the keys and then turn your Mac back on using the power button.

(From https://www.idownloadblog.com/2016/05/23/mac-startup-key-combinations/ see also: https://support.apple.com/en-us/HT201255)