What is the difference between the passé composé and the imparfait tenses?

The passé composé is the most commonly used tense to refer to actions completed in the past. They tend to be one off, completed actions. For example, hier, j'ai mangé un gâteau.The 'imparfait has two main uses: to describe on-going actions and states of being in the past, and to state habitual actions in the past. The imparfait is used to describe people, places, conditions or situations in the past. The imparfait is also used to state habitual actions in the past. These past habits are often translated as 'used to, or 'would.' For example, quand j'étais petite, je mangeais des bonbons chaque jour. Some verbs occur more frequently in the imparfait when they are in the past since they typically describe states of being: être, avoir, vouloir, pouvoir. These verbs sometimes occur in the passé composé, it depends on the context of the sentence and what you are trying to say.