The indefinite article a, an, the

A (or An)
- used with countable nouns to indicate one.
- is not used before a plural noun.
- is not used before uncountable nouns.
- when talking about cost, speed, how often when we do something.

The - used when a word is used a second time.
- used when only one object exists such as the sun, the earth.
- used before names of seas, rivers, groups of islands or mountains, kingdoms, republics, deserts, plural names of countries such as the Nile, the United States of America etc.
- usually say the before the places we visit in a town.
- with musical instruments when we talk about playing them or listening to them.
- with listening to the radio. No article with watch television.

No article or the
- before names of places. A or The is not usually used before names of villages, towns, streets, cities, countries or continents.

- before school, college, university, work, home, bed, town, hospital, church, prison. Howevery, we only use a or the before these words when the building is important and not its use.

- before names of academic subjects, languages, sports, meals.

Reference: Grammar Practice for Pre-Intermediate Students.