Modal verbs - i verbi modali

http://www.englisch-hilfen.de/en/grammar/hilfsverben2.htm

http://www.english-4u.de/modal_verbs1.htm

http://www.english-4u.de/modal_verbs_ex1.htm

http://www.nspeak.com/newbasic/grammatica/modalican.htm

http://www.englishclub.com/grammar/verbs-modals_can_1.htm

http://learnenglish.britishcouncil.org/en/english-grammar/verbs/modal-verbs

Present perfect vs past simple

https://www.engvid.com/present-perfect-or-past-simple/ 

https://www.youtube.com/watch?v=AM3RydRXn_g

https://www.youtube.com/watch?v=eA2_i2yDaAo

http://www.perfect-english-grammar.com/present-perfect-or-past-simple.html

https://www.ego4u.com/en/cram-up/grammar/simpas-preper

http://www.learnenglishfeelgood.com/simple-past-present-perfect1.html

https://www.ego4u.com/en/cram-up/grammar/tenses