http://baladre.info/english/sedaviwebfront/preperfwords.htm
http://www.esl-lounge.com/student/grammar/2g7-just-yet-already.php
http://www.montsemorales.com/gramatica/PresPerfYetAlrd.htm
http://www.montsemorales.com/gramatica/PresPerfpartic.htm
http://baladre.info/english/sedaviwebfront/presperfwordsgap+grammar.htm