Resolved questions
How do I know when to use the preterite or the imperfect in Spanish?

"Yo hablé" is used when an action started and finished in the past, like

"I called you this morning, but you did't pick up the phone"
Te llamé esta mañana, pero no contestaste

Yesterday I ran into him on the street.
Ayer lo encontré en la calle. (it happened once)


.........
"Yo hablaba" is used when the action lasted for a period of time, it is repeated or a habit, like

She seemed to be nice, but she talked too much. (during dinner, for example)
Ella parecía ser agradable, pero hablaba mucho

She didn't talk to me when we were studying Highschool
Ella no me hablaba cuando estábamos en preparatoria (it lasted years)

The next ones are a little bit harder to explain because the translation doesn't match exaclty and there are two ways to say the same thing in spanish. The first one is literal

I used to call her every day.
Solía llamarle todos los días

The second one is the same in english, it´s written differently in spanish, but it means the same:

I used to call her every day
Yo le llamaba todos los días

When I was a kid I used to enjoy our trips to the beach
Cuando era niño solía disfrutar nuestros viajes a la playa
Cuando era niño disfrutaba nuestros viajes a la playa (them both mean the same)


More examples:

We used to eat dinner at the same restaurant every once a week.
Nosotros solíamos comer en el mismo restaurante una vez a la semana
Nosotros comíamos en el mismo restaurante una vez a la semana (repetition or habit)

Yesterday we ate at the same restaurant
Ayer comimos en el mismo restaurante

.....
One way to know the difference is by words like always, often, everyday, etc. you have to use "hablaba" with those words.

And with words like yesterday, last night, last week, etc. you have to use "hablé"

I hope I made my self clear :)