When should I choose to use the tenses of the "imparfait" or the "passé simple"?

These two tenses both express actions and states in the past. However, the imparfait should be used in cases of repetitive situations, a description or an action without a limited time frame. For instance, "everyday I woke up at dawn" should be translated using the imparfait. On the other hand, the passé simple is used in cases of a sudden or unexpected event, or to express a finished action. Quite often these two tenses are combined within a sentence to add impact to an idea. For instance, in the sentence "She was walking in the park when she saw a sparrow.", the fact of seeing the sparrow is emphasized by the previous use of the imparfait.