One of the things people find the hardest when learning languages is not forming the tenses but when to use the preterite or imperfect when talking about events which happened in the past. The most important thing to remember is that if the event was a one time occurance and is now finished, the preterite is used. If you are talking about an event which regularly happened in the past, the imperfect is needed.
1. The Preterite
- Used for events that took place in the past and are now complete.
- Describes actions or occurrences that took place and were completed in the past:
E.G. Me levanté, me lavé y salí. (I got up, I washed and went out)
Nicolás me llamó cinco veces anoche. (Nicholas called me five times last night)
- Also refers to states of affairs that lasted during specified (possibly very long) time limits in the past and have now ended. * Often the prepositions durante or hasta signal this usage.
E.G. Felipe González fue presidente del gobierno durante nueve años. (Felipe was president of the government for nine years.)
Ella se quedó allí hasta el lunes por la mañana. (She stayed there until Monday morning.)
2. The imperfect tense
- Describes actions/events that were in progress in the past (was-ing or were-ing)
- Describes actions/events that were habitual or customary in the past (used to).
- Beginning and end of these occurrences is unspecified.
E.G. Alberto y Juan se reían. (Alberto and Juan were laughing.)
Me caí mientras bajaba la escalera. (I felt whilst I was going down the stairs)
Ibamos todos los jueves al cine. (We used to go to the cinema every Thursday.)
Now, try and do these examples yourself:
1. María used to work at Zara.
2. He always arrived late.
3. We frequently travelled first class (en primera clase.)
4. I was sleeping when he arrived.
5. We didn't want to go to the shop.
I hope this makes the differentiaion clearer. After a lot of practice, both tenses will come naturally without too much thought!