Frequency adverbs

audio explanation

http://www.inglestotal.com/adverbs-of-frequency-adverbios-de-frecuencia-lesson-13/

aquarium explanation
http://esl.about.com/library/grammar/blgr_adverbs_frequency.htm

exercise on line
http://perso.wanadoo.es/autoenglish/gr.frequ.i.htm

printable exercise
http://perso.wanadoo.es/autoenglish/gr.frequ.p.htm