Same noun used with all three articles conveys different meaning.
- He is at a school. (We don't know which school.)
- He is at the school. (We know which school.)
- He is at school. (He is a teacher or student and is teaching or learning.)
- I buy the paper every day. (Newspaper)
- The professor presented a paper on recent findings about cancer cells. (A piece of writing)
- The children are learning the art of folding paper into decorative objects. (Material)
It helps to observe the following.
(1) When we use two or more adjectives to describe the same person or thing, we use the article only before the first adjective.
- He talked to a tall and blonde woman.
(He talked to a [one] woman who was tall and blonde.)
(2) When we use two or more adjectives to refer to more than one person or thing, we use the article before each adjective.
- He talked to a tall and a blonde woman.
(He talked to two women, one of whom was tall and the other was a blonde.