When to use the imparfait and the passé composé?

Both the imparfait and the passé composé are tenses that describe a past action. The passé composé is used to refer to completed events in the past e.g. j'ai mangé (I ate - indicating that you were eating in the past and have since completed this action). The passé composé can be used to show a link between a past event and the present - j'ai mangé can also be translated as 'I have been eating', 'I have eaten'. The imparfait, however, is used to refer to ongoing, continuous or incomplete actions in the past. Je mangeais for example conveys the ongoing nature of a past action. Instead of 'I ate' (which would be used with the passé composé), the imperfect is used to say 'I was eating'. The imparfait can also be used to describe a continuous habit in the past e.g Quand j'étais petite, je mangeais une pomme chaque jour (when I was little, I used to eat an apple every day).