Q: iMac wireless keyboard letters t,g,b,- only work when pressed with fn key

Hello markiskiwi,

Thanks for using Apple Support Communities.

For assistance with keys on your Apple wireless keyboard not functioning correctly, I suggest that you follow through the steps outlined below.

Your keyboard has one or more keys that don't respond

Use the Keyboard Viewer to test whether the keyboard keys are responding correctly when they are pressed.

1. Choose System Preferences from the Apple menu, then select Language & Text.
2. Select Input Sources.
3. Select the Keyboard & Character Viewer checkbox.
4. Enable the input source for the keyboard layout you're testing by selecting the checkbox next to it.
5. Make sure that "Show Input menu in menu bar" is enabled.
6. Choose Show Keyboard Viewer from the Input menu in the menu bar.  
7. The Keyboard Viewer showing the keyboard layout appears on your display.
8. Type the key on the keyboard that doesn’t respond and see if the corresponding key highlights on the Keyboard Viewer. If it does, that key is functioning correctly.

Notes:

• If the mouse keys feature in the Universal Access System Preferences pane is enabled, many of the keyboard keys may not respond as expected.
• If issues persist, try starting up your Mac in single-user mode to see whether a software issue is causing the issue.

If your Apple wireless mouse, keyboard, or trackpad aren‘t working as expected

Take care.