Since or For ?
Here are some good links :
http://teacher.schule.at/tom-schoftner/drw_files/for_since_cloze1.htm
http://home.eduhi.at/teacher/nausner/members/grammar/forsince/forsince1.htm
http://www.padl.ac.at/class21/Englisch/grammar/preperf/forsince2.htm
http://a4esl.org/q/h/lb/forsince.html
http://www.better-english.com/grammar/forsince.htm
http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/upperf1.htm