Yes, I tried pressing the shift key while my computer is starting, but it's really weird because nothing happens. I don't know if I'm doing it incorrect, like, there's a certain moment to press shift or if it's just my computer going crazy. The thing is, I can perform a SMC reset. Which is why I don't understand why I can't start up in Safe Mode if it recognizes other commands.
Resetting the SMC on a White MacBook doesn't require the keyboard so I'm not sure what you're doing. Your MacBook DOES have a battery that you can remove correct? If so here's the procedure
To reset the SMC
Shut down the computer.
Disconnect the MagSafe power adapter from the computer, if it's connected.
Remove the battery.
Press and hold the power button for 5 seconds.
Release the power button.
Reconnect the battery and MagSafe power adapter.
Press the power button to turn on the computer
Thank you but I can't remove my battery, and according to this http://support.apple.com/kb/HT3964 I have to press the shift-control-option keys and the power button at the same time, I don't want to reset the SMC anyway. The thing is, I can't start up my computer, it stays on the grey screen with the apple logo and the spining gear. I consulted the Apple support page and said that I have start up in Safe Boot or reset the NVRAM/PRAM. The problem is, I can't perform either of them. I've also tried to start up in single-user and verbose mode, but nothing happens when I press the keys. It's really weird, because, I tried to reset the SMC to see if the keyboard wasn't working, but it was, I succesfully reseted the SMC, so, I don't know what to do or what's wrong. Anyway, thanks for your help!!