http://www.helpforenglish.cz/anglictina-pro-zacatecniky/c2008030602-prislovce-urcujici-frekvenci.html
http://www.englisch-hilfen.de/en/exercises/adjectives_adverbs/adverbs_of_frequency.htm
http://www.englisch-hilfen.de/en/exercises/adjectives_adverbs/adverbs_of_frequency1.htm