Explicação com vídeo: https://www.youtube.com/watch?v=zCJUJVlYLhk
Link 1: http://www.language-worksheets.com/adverbs-frequency-elementary.html
Link 2: http://www.autoenglish.org/basic/gr.frequ.pdf
Link 3: http://www.grammarbank.com/adverbs-of-frequency.html
