AFAIK the canonical FPS setup is wasd for up/strafe left/down/strafe right, left click for fire, right click for secondary fire, x for duck and z for prone. Or, anyway, it's what I use. I think it allows for more flexibility in movement, but sometimes cheap keyboards don't support strafe+forward+duck together too well without using modifiers (ctrl/shift).

Actually shifting wasd over to the right is a more natural stance if you do any typing. WSAD => EDSF etc. If you're not already used to wasd then definitely consider it.

It is also useful to have an extra set of keys to the left other than tab/caps/shift

Interesting. If only I'd been shown that 15 years ago when I still had some ability to rewire my neuronal pathways.

I can't imagine being too old to get used to a new control scheme. It's like being too old to switch to standard instead of automatic.

It is more like a well established touch typer switching from qwerty to dvorak, it can happen just takes will to change and a good amount of time.

how do you walk backward?

Mouselook around, or whip it around if you're playing an FPS. I can't think of too many times where actually walking backwards would be advantageous since it usually makes you move more slowly.

shooting while retreating would be one scenario I can think of. Someone else said it but the typical fps setup is wsad space jump, ctrl or shift =crouch or run with mouselook.

Alt key, right under my left thumb.