I have studied my subject for years, yet there is still always more to learn.
There is teachers above me who have spent their life studying and they say they don't know.

I think if I said I knew my subject, then it would become boring, as I know it.

Its like playing the guitar, you can always learn 'knew'(ask yODa) tricks, even if you were the best in the world, you could still learn a 'knew' trick off a amuter, who had taught them selfs and so learned a diffrent way to play.

exactly, you only don't know what you don't know... anything you can think about/do/act... you must already know... or have just brought into knowledge in the act of thinking/doing.