il passato prossimo vs. l'imperfetto
http://www.youtube.com/watch?v=3m_PhT6UjBs - describe differences
http://quizlet.com/5457340/when-to-use-passato-prossimo-vs-imperfetto-flash-cards/ - key terms between each tense
http://www.uvm.edu/~cmazzoni/3grammatica/grammatica/passatoprossimoimperfetto.html - grammar explanation
http://homes.chass.utoronto.ca/~ngargano/corsi/100/wk18.pass.imp.html - you try practice!