When do you use the "Imparfait" as opposed to the "Passé-Composé" in French?

Both the imperfect and perfect tense evoke actions in the past, however they are used differently.The imperfect is used to describe something or someone, to talk about a habit (an action that was done over and over), or an action in progress. For example, "Quand il faisait froid, il n'oubliait pas de mettre ses gants sur la cheminée.""Quand il faisait froid" describes the weather, and "il n'oubliait pas" is a repeated action.
The perfect tense is used to describe a precise and completed action. For example, "Il a mis ses gants sur la cheminée en rentrant du travail." "Il a mis ses gants" is an isolated action in the past, therfore it requires the perfect tense. Words introducing a single action or a string of single actions are indicators that you should use the perfect tense. For example, "soudain", "brusquement" or "ensuite" are temporal indicators that introduce an action.