http://www.aprende-gratis.com/ingles/curso.php?lec=presente-continuo
http://www.inglestotal.com/present-continuous-progressive-tense-uso-y-estructura-to-be-ing-el-gerundio-lesson-8/ **
http://madridteacher.com/Grammar/presente-continuo.htm **
http://www.mansioningles.com/gram39.htm
http://www.helloworld.com.es/Spanish/quick%20reference/verbs/english/presentcontinuous.htm
http://html.rincondelvago.com/presente-continuo.html
http://www.curso-ingles.com/gramatica-inglesa/pcontin.php
http://www.languageguide.org/english/grammar/esp/part1/present_progressive.jsp **