Examples:

• 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)

Examples:

(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.