When Keys Don't Behave Correctly

When keys on the keyboard do not do what you expect, a first step should be to check all the MacOS settings which can modify key behavior.  These settings sometimes change for no explainable reason.  The most common examples seem to be finding that Mouse Keys (which makes 789uiojkl unusable) or Full Keyboard Access (which can stop the Space Bar from working in certain apps) are suddenly ON when they should be OFF.

Here is a list of places to check for mistaken settings:

System Settings > Keyboard  >  Key Repeat Rate, Delay Until Repeat

(Wrong settings can kill the Accent Menu)

System Settings > Keyboard > Press fn key to

(Can change what that key does)

System Settings > Keyboard > Keyboard Navigation > Keyboard Shortcuts

(Can change behavior of F keys, modifier keys, and various combinations used for menu shortcuts)

System Settings > Keyboard > Text Input > Input Sources > Edit 

(Wrong language keyboard will change characters which letter keys produce)

System Settings > Keyboard > Text Input  > Text Replacements

(Letter sequences can be replaced automatically by different ones)

System Settings > Accessibility > Motor > Keyboard  > Full Keyboard Access > ⓘ > Commands

(Can make some keys non-functioning or behave strangely or generate colored rectangles )

System Settings > Accessibility > Motor > Keyboard  > Sticky Keys

(Will keep modifier keys on even after you have stopped pressing them)

System Settings > Accessibility > Motor > Keyboard  > Slow Keys

(Will require holding down the key for some time before it does anything)

System Settings >  Accessibility > Motor > Pointer Control > Mouse Keys

(Can disable 789uiojkl or number pad)

System Settings > Desktop & Dock > Mission Control > Shortcuts

(Can change behavior of Function Keys, Fn key,  and left or right modifier keys)