How do I know when to use the perfect tense and when to use the imperfect tense?

The perfect tense is used to describe a completed action that happened in the past e.g. 'hier, nous avons joué au foot'. This action is no longer happening and can be seen as an event that can be 'ticked off'. Also, the reference to time (hier) further shows that it is completed. The imperfect tense, however, describes ongoing events without reference to a start or finish time e.g. 'il était tard quand nous avons joué au foot'. It can also describe a habitual action in the past, being translated as something you 'used to' do or 'would' do e.g. 'Mme Fontaine lisait aux enfants quand elle avait le temps'.