CAN/ COULD

http://autoenglish.en.wanadoo.es/gr.can.i.htm
http://www.theyellowpencil.com/gramacan.html
http://www.englishclub.com/grammar/verbs-modals_can_quiz.htm
http://www.ompersonal.com.ar/omexercise/verbosmodales/unit15.htm
http://esl.about.com/library/quiz/blgrquiz_canbeable.htm