Today I bought a MacBook Air with OS X 10.9.4. I have problem with account password. I can log in normally at start, when the screen turns off I get unlock screen, but I can't unlock it and I am 100% sure, that I type correct password. I also can't unlock preferences and do stuff with 'sudo' in terminal. Does anyone know how to solve this?
You should be able to reset it using one of the following methods.
Method 1: > system preference > users & groups
if the check box beside "allow user to reset password using Apple ID is checked, you should be able to reset it using you apple id and password (make sure you're connected to the internet)
Method 2: else you can boot into the recovery partition by holding CMD & R, then on the top menu you should be able to navigate and find an option that says "reset password", the down side is you lose all you keychain items (your WIFI passwords, safari saved passwords, etc)