can and could

https://learnenglish.britishcouncil.org/es/english-grammar/verbs/modal-verbs/can-or-could
http://www.englishexercises.org/makeagame/viewgame.asp?id=3826
http://mbonillo.xavierre.com/exercises/eso2/2esowint09/cancould2eso3.htm