As time goes on, you'll find that you forget a LOT of what you learn. The important thing to learn is where to find the information. To this day, I still have to look up commands in the manual (especially PHP), because I can't remember the order of the parameters, or I forget what value is returned (FALSE? An array?).
PHP makes this even worse by reversing the order of parameters, even among similar functions, or adding an _ in a command name in some places, and not others.
The bottom line is, learn where to look for the info, and you'll be a pro in no time.