I can't believe no one has ever asked this, but I can't seem to find it:
How long should you hold down key combinations, such as Command + S, while your Mac is booting? I can get this iMac to boot into safe mode (sometimes), but not Single User, Verbose, or Diagnostic modes, which seems strange to me, thus I assume I'm not doing something correctly.
Do you just hold the keys briefly or hold the keys for longer? If the latter, how long exactly? This iMac just has that gray boot screen, and there's no indication that anything is happening, so I have no way of knowing when I should stop pressing the key combos or if pressing them is even doing anything.