After incurring some slight liquid damage and ceasing function, we replaced the keyboard on an early 2011 Macbook Pro. Nothing else is compromised–using an external keyboard and mouse allows one to use the computer. We replaced the keyboard with a new one and now it works, but only at the initial boot screen, allowing us to reset the PRAM, the SMC, and start up in verbose mode and safety mode–however afterward it loses functionality. Both the trackpad and keyboard don't work once Yosemite is launched, but the keyboard does prior to login. Plugging in external hardware resolves the issue again but we want the laptop's keyboard to work. Could this be a software/driver issue of some kind (We're running OS10.10.04)? The keyboard is of the same period as the Macbook and should work natively, though it is a used product... Issues could also have resulted from any mistakes during reassembly, but because the keys work on startup we may be able to rule out personal error. Any ideas would be helpful.