When would you use the <<imparfait>> and when would you use the <<Passé Composé>>?

Both of these tenses are used when speaking about an action in the past. The imparfait is appropriate when the action may still be being completed or was being done over a certain time period. For example "je jouais du piano quand j'étais jeune"means it was happening for a while. Also, this could imply that I still play the piano. However, if an action has been completed and it is attributed to a specific time frame such as "A dix-sept heures" then the Passé composé is appropriate. An auxilary verb and the past participle are therefore needed as the action is defininitely complete. e.g. " A dix-sept heures, j'ai fait la vaisselle".