There is quite a bit on the forum about article usage. The question has been asked and answered many times.
Not a professional teacher
I'm not a teacher. I speak American English. I've tutored writing at the University of Southern Maine and have done a good deal of copy editing and writing, occasionally for publication.
I am not here for speaking. I am here purely for grammar/formal/academic writing. So I am trying to find rules that explain different cases so I understand better and not forget.
It doesn't matter that you don't want to improve your spoken English. Reading how native speakers write, and listening to how they speak, remain the best ways of getting to grips with it. However, as Charlie said, it's not a quick process. It can, and probably will, take years to really get to grips with it.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.