1 http://www.helpforenglish.cz/article/2006062401-minula-modalni-slovesa-cviceni-1
2 http://www.helpforenglish.cz/article/2006042201-intermediate-grammar-test-2
3 http://elt.oup.com/student/headway/int/b_vocabulary/unit11/hwy_int_unit11_6?cc=cz&selLanguage=cs