How do I know when to use the imperfect tense when talking about the past?

The imperfect tense is used for actions that: a) occurred repeatedly i.e. I used to do ballet twice a week, when I was younger. b) actions that occurred over a period of time i.e. I used to drink alcohol. Additionally, this tense can be used to tell a story of an event that occurred in the past in a "were/when" structure, i.e. We were enjoying the party when the police arrived. Finally, the imperfect is often used for feelings and mental actions i.e. We were feeling sad. The important thing to remember is when using the past tense in Spanish, ask yourself:

1. Did this action happen repeatedly?
2. Did this action take place over a period of time? If either of the answers is yes, use the imperfect, otherwise the preterite is the tense you're looking for!