I have an older Macbook Pro, 15" that I picked up on eBay. I had been letting my mother use it for the past year.
Somehow she (or someone else) changed her password. Now she uses an external keyboard (USB, Apple made) because sometime ago the I and K keys stopped working on the built in one. Going into single user mode to reset the password I noticed that while I can hold Apple+S on the external keyboard to boot into it, it won't let me type through it.
There are a series of commands to mount the operating system and change the password for a user, all use an I or K.
I've tried another USB keyboard no effect.