liba.kubalakova.sweb.cz/SKOLA/englishweb/GRAMMAR/Frequency%20adverbs/fr1.htm
http://www2.arnes.si/~oskplucija4/ces/howoften.htm
http://jerome.boulinguez.free.fr/english/file/hotpotatoes/adverbesfrequence.htm
http://www.englisch-hilfen.de/en/exercises/adjectives_adverbs/adverbs_of_frequency.htm