The Preterite is used for past actions that are completed.
Conjugating Verbs in the Preterite Tense
The endings that are used to conjugate regular –ar and –er/ir verbs are:
-ar: yo- é, tú- aste, él/élla/usted- ó, nostros- amos, ustedes/éllos/éllas- aron
-er/-ir: yo- í, tú- iste, él/élla/usted- ió, nostros- imos, ustedes/éllos/éllas- ieron
Examples:
1. Le mandé un mensaje a Rosa.
I sent a message to Rosa.
2. Sanó el movil en medio de la clase.
The cell phone rang in the middle of class.
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/