In French, when telling a story in the past, you need to choose between two tenses: the imparfait and the passé composé. The imparfait sets the scene and describes background information, like what was happening or how things were. The passé composé describes the main, completed actions or events of the story.

Let's look at interrupting an ongoing action. Use the imparfait for the background action that was in progress, and the passé composé for the specific event that happened. For example, 'Je marchais quand j’ai vu Paul' means 'I was walking when I saw Paul'. The walking was the background, and seeing Paul was the event.