THEORY
http://www.curso-ingles.com/aprender/cursos/nivel-avanzado/verb-tenses-present-perfect/present-perfect
ACTIVITIES
1. http://www.agendaweb.org/exercises/verbs/present-perfect
2. http://www.nspeak.com/newbasic/grammatica/presperfect.htm
3. http://www.english-grammar-lessons.com/irregularverbs/exercise6.swf
4. http://perso.ya.com/mjandres/archivos/presentperfect1.htm
