Present continuos exercises

http://perso.wanadoo.es/autoenglish/gr.prescon2.i.htm

Past continous explanations

http://www.edufind.com/english/grammar/Tenses10.cfm

Past comtinous exercises

http://perso.wanadoo.es/autoenglish/gr.pascon.i.htm

Simple past exercises

http://perso.wanadoo.es/autoenglish/gr.pastsim.i.htm

http://perso.wanadoo.es/autoenglish/cros1.htm

http://perso.wanadoo.es/autoenglish/cros2.htm

http://perso.wanadoo.es/autoenglish/cros3.htm

Present perfect exercises

http://perso.wanadoo.es/autoenglish/gr.since.i.htm

Passive Voice explanation

https://docs.google.com/present/edit?id=0AZD8asPZeCelZGNrZjRoMjRfMzM5Y2M4YmZ0dDU&hl=es

Simple past explanation

http://www.edufind.com/english/grammar/Tenses9.cfm