1.  The preterite tells you about an action that started and ended at a definite time.  Ocurrió un temblor = An earthquake occurred.  Muchos edificios se cayeron = Many buildings fell down.

2. The imperfect tells about past actions without saying when they begin or end. The action isn’t “perfectly done,” therefore it is imperfect.  Todo el mundo dormía = Everybody was sleeping / slept (continuously in the past).  La tierra temblaba. = The earth was shaking / shook continuously.  Había gente por todas partes = There were people everywhere.

3.  Nos enamoramos cuando éramos jóvenes.  (we fell in love [you see it happen once]  when  we were young [long period of time being young] )

4.  Mientras yo estudiaba, sonó el teléfono.  While I was studying, the phone rang. When I was little, I broke my favorite toy. Cuando era__ niña, _rompí___ mi juguete favorito.

5. **The Word “mientras” will be useful to remember as a trigger for the imperfect tense. While they danced, the servers brought cake. Mientras bailaban, los meseros trajeron pastel.

7.  http://www.quia.com/pop/62615.html?AP_r and=602620746