Diagnostic, Recovery, Safe, Startup Manager, Verbose, reset SMC/NVRAM, firmware restore, Target Disk/Display, Single-User (SUM) in outline.
Safe mode
Disk integrity checks appear to have been dropped from Safe Boot. OpenDirectory databases are rebuilt. With a full listing of all blocked extensions.
Start up in Safe Mode: should it start by checking and repairing your boot disk, or should you do that separately in Recovery Mode?
When should you reset your SMC and NVRAM, repair permissions, install a Combo update, or re-install the whole of macOS?
Safe mode, or Recovery? Which type of Recovery? How to engage all those useful modes at startup, and when shutting down.
Start up in Safe mode, and it’s stable; restart in ordinary mode, and your Mac is in trouble. How to identify and fix the problem.
Another commonly-suggested panacea, Safe mode has its uses, if you understand why it is ‘safe’.
All the useful key combinations, arranged by function rather than keys.
What to do when your Mac won’t start up, and is acting dead, or at least mortally wounded.
When I started my iMac up, the fan has run up to be really loud. How can I fix this?
The Eclectic Light Company 