Explicação com vídeo: https://www.youtube.com/watch?v=mqzlf9RXNkA
Link 1: http://first-english.org/english_learning/english_for_beginners.htm#indefinite_articles
Link 2: http://www.grammarbank.com/indefinite-articles.html
Link 3: http://www.eslpdf.com/esl-pdfs/articles_aan1.pdf
