Preterite vs. Imperfect

Understanding the difference between the preterite and the imperfect can be a difficult grammar topic to master, specifically because English doesn't distinguish between these two verb tenses. Generally, the preterite is reserved for actions that are finite i.e. they begin and end in the sentence: "Ayer fui al supermercado." - "Yesterday I went to the supermarket."

The imperfect is used for actions that are repeated in the past, and have no specific start or end: "Como niño, mi abuela me llevaba al parque." - "As a kid, my grandmother would take me to the park."   

Here's an article that will give you some more information on the distinction between the imperfect past and the preterite:

http://www.spanishdict.com/topics/show/64

It takes some time to get used to using these tenses, but don't worry -- practice makes perfect!