Modal verbs

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

https://www.curso-ingles.com/aprender/cursos/nivel-avanzado/verb-tenses-present-perfect/modal-perfects

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