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

First and foremost, the tenses in French can never be translated literally from English! Whether the phrase 'I was walking' is translated into PC or imparfait is determined by the context of the phrase in the sentence or story.In about 90% of the cases, the PC is the past tense used, as it describes a finished action or event ( i.e. not ongoing ). The imparfait, however, is a bit more tricky to use as there are a set of rules of when it is used. These always include: When a regular action is described that happened in the past such as 'Je faisais mes devoirs chaque jour', When a past feeling is being described, such as 'Je détestais mes camarades' , When the end point of the action is unclear, such as 'Je resterais à l'école', Finally, when it sets an interrupted background to the story, such as ‘Je faisais mess devoirs quand …’ In the last case, the situation is a bit tricky as the second clause of the sentence with an "interruption" is always in PC, as shown in the example : Je faisais mes devoirs quand j'ai décidé de revenir.  Here you can see that the first clause is in imperfect & the second one is in PC.