What is the difference.../ Que es la (el?) differencia...

[ZOMGSumi]

Between el preterito and el imperfecto in past tense?
For example, hablaste vs. hablabas.

I have never understood the difference between the two tenses. My teacher explained the second one as an action that was interrupted. Somthing like...
We ate dinner when my grandma called.
Comíamos la cena cuando llamó mi abuela.

I'm not entirely sure if that is right. Some clarification would be wonderful.  

[Nivens McTwisp]

Preterite is used for a specific event that happened once and is no longer, while Imperfect is used for something that used to be.
Basically, any time you would say "used to (verb)" in English, it translates to Imperfect in Spanish.

Por ejemplo...
Yo tuve una manzana, pero de repente mi amigo la comió y ahora le odio.
I had an apple, but then my friend ate it and now I hate him.

Cuando era pequeño, tenía muchos amigos pero ahora todos creen que estoy loco.
When I (used to be) was little, I (used to have) a lot of friends, but now everyone thinks I'm crazy.

Espero que este te ayude.

 

[ZOMGSumi]

Okay, that makes sense. Thank you~

I remember there being portions of my Spanish tests that had, say, something like a short story. And parts would be in imperfecto and others would be in preterito.
Should I just guess whether the past action is something that "used to be"?