a = indefinite article (not a specific object, one of a number of the same objects) with consonants She has a dog. I work in a factory.
an = indefinite article (not a specific object, one of a number of the same objects) with vowels (a,e,i,o,u) Can I have an apple? She is an English teacher.
the = definite article (a specific object that both the person speaking and the listener know) The car over there is fast. The teacher is very good, isn't he?
The first time you speak of something use "a or an", the next time you repeat that object use "the". I live in a house. The house is quite old and has four bedrooms. I ate in a Chinese restaurant. The restaurant was very good.
DO NOT use an article with countries, states, counties or provinces, lakes and mountains except when the country is a collection of states such as "The United States". He lives in Washington near Mount Rainier. They live in northern British Columbia.
Use an article with bodies of water, oceans and seas - My country borders on the Pacific Ocean
DO NOT use an article when you are speaking about things in general I like Russian tea. She likes reading books.
DO NOT use an article when you are speaking about meals, places, and transport He has breakfast at home. I go to university. He comes to work by taxi.