The use of English articles is almost always confusing to Vietnamese learners. This is a summary of the use of a, an, the from British Council ‘Learn English’ website, with exercises. Hope it can help you:

Definite article ‘the’: http://learnenglish.britishcouncil.org/en/english-grammar/determiners-and-quantifiers/definite-article

Indefinite articles ‘a’, ‘an’: http://learnenglish.britishcouncil.org/en/english-grammar/determiners-and-quantifiers/indefinite-article-and