"He is in THE office." (On jest w biurze.) vs. "He is in office." NO article, meaning "He HOLDS [elected] office." (On jest urzędnikiem/posłem...)
the knowledge of the distinction is taught so late in the curriculum a lot of people never reach that level (I was taught that in high school but I had intensive English course in high school - 6hours per week)
- despite the fact that I have learned English for seven years and that I use it quite a lot I still make mistakes when it comes to articles