How do I know whether to use the imperfect or the preterite tense?

The imperfect tense is used to describe actions that do not have a clear beginning and end. For example, actions that were repeated (‘I used to go to school’), or, what we could call ‘background information’, (‘the house was white’).The preterite tense, on the other hand, describes actions that are complete and finished, with a clear beginning and end (‘he ran away’, or ‘she threw the ball’).