There are certain subjects that when talked about in the past should use the Imperfect tense only. These catagories include:
Physical conditions
Location
Emotional state
Appearance
Dates
Weather
Age
Age/desires
Time
Examples:
1. Yo estudiaba en la computadora a la escuala.
I studied on the computer at the school.
2. Yo no vivÍa con no movil.
I could not live with no cell phone.
http://www.studyspanish.com
http://www.rocketlanguages.com/spanish/imperfect_spanish_past.php
http://www.123teachme.com/learn_spanish/imperfect_tense_2
http://www.bellaonline.com/articles/art32108.asp
http://www.dummies.com/how-to/content/expressing-uncertainty-in-spanish-with-the-imperfe.html
http://spanish.about.com/od/verbtenses/a/two_past_tenses.htm
http://www.onlinespanishhelp.com/
http://www.drlemon.net/Grammar/pretvsimp.html
http://www.learn-spanish-help.com/
http://www.spanishclassonline.com/