There are in all 2 articles and three words that represent the articles. They are "a" "an" and "the". The articles a/an are indefinite because they stand for one in the sense of any. An is used before the words starting in a, e , i , o, u. And the article "the" is used for something already mentioned, as in "the book on the table" also when a singular noun is meant to represent a whole class as in "the donkey is a beast of burden".
Also used with superlatives eg: The best fan in the market.
All republics, islands, oceans, seas, canals take the before them, eg: The united states, the British Isles.