jay, articles are really hard - sometimes you just have to have read a lot to have a feel for how they are used.
Even today, there are times when I struggle with whether to include one or omit it - and English is my native language and I write for a living. So don't feel bad that it is taking you some time to understand.
A key part of this is understanding countable and uncounatable nouns. How confident do you feel about that?