Past continuous

https://learnenglish.britishcouncil.org/en/english-grammar/verbs/past-tense/past-continuous

https://www.google.it/search?q=past+continuous&safe=active&client=safari&hl=it-it&prmd=ivsn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwijtu61iO3QAhWBXhQKHcXaCxAQ_AUIBygB&biw=768&bih=928&dpr=2#imgrc=pGOSLT1wC5sBpM%3A

More practice: present perfect vs simple past

http://www.esl-lounge.com/student/grammar/2g11-past-simple-present-perfect-gap-fill.php

http://www.agendaweb.org/verbs/present-perfect-simple-past-simple-exercises.html

http://www.perfect-english-grammar.com/support-files/present_perfect_or_past_simple_2.pdf

http://www.grammarbank.com/present-perfect-exercises.html