When should I use passé composé and when should I use the imperfect tense?

The passé composé is always used once an action is completed. For example, when saying 'yesterday I went to school', you would use the passé composé - 'Hier, je suis allée au collège'. The imperfect tense is usually used if the action has not been completed and is still ongoing/reoccurring. For example, 'I was going going to school', you would use the imperfect tense - 'J'allais au collège'.