What is a unnecessary shift, necessary shift, and a no shift?
Check these sites for shifts in subjects and/or verb tenses.
http://bcs.bedfordstmartins.com/easywriter3e/20errors/10.asp
http://wps.ablongman.com/long_faigley_penguinhb_1/0,7325,506544-,00.html
http://www.wicknet.org/english/etillman/warriners/warriners_16.htm