You can learn more about "modal verbs" on these websites. You can also do some interesting activities:
http://www.bbc.co.uk/learningenglish/english/search?q=modal+verbs&submit=GO
http://www.englishclub.com/grammar/verbs-modals.htm
http://www.englishpage.com/modals/interactivemodal1.htm
http://ntic.educacion.es/w3//eos/MaterialesEducativos/mem2001/ienglish/index.html
http://www.usingenglish.com/handouts/398.html
http://www.usingenglish.com/handouts/528.html
http://web2.uvcs.uvic.ca/courses/elc/studyzone/330/grammar/330-modals-ability.htm