Completed event
Use passé composé for a completed event that moves a story forward: Le bus est arrivé à huit heures.
Background and habit
Use imparfait for descriptions, repeated habits and ongoing background actions: Il pleuvait et les voyageurs attendaient.
Use them together
A background action can be interrupted by a completed event: Je lisais quand le téléphone a sonné.
Memory check
Ask whether the action is the scene or the event. The scene often uses imparfait; the event often uses passé composé.
Le_Taxi preparation resources are educational. Confirm current TEF, TCF and immigration rules through their official organizations.