6. Articles — a / an / the

• a / an — one of many, or first mention.
  I saw a dog. (some dog, not specific)
• the — specific, already known to both speakers.
  The dog was black. (the same dog we just mentioned)
• No article — general plural or uncountable ideas.
  Dogs are loyal. I like music.

Cheat: first time = a/an, next time = the.