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

These are both common tenses used to describe past actions in French. The imparfait (imperfect) tense has endings like this: Je -ais, Tu -ais, Il/Elle/On -ait, Nous -ions, Vous -iez, Ils/Elles -aient, while the passé composé is composed (like the name) of two different parts, namely, an auxiliary verb and a past participle, e.g.: Je suis allé(e), tu as mangé.It can be tricky to know which one to use as it doesn't quite add up to what we use in English. However, as a general rule, the imperfect is used to describe the following actions:Actions taking place over a long period of time (e.g. the Nazi occupation of France - quand les Nazis occupaient la France...)Conditions (the 'setting' of a description, e.g. someone's age, the weather, something that was happening while another, shorter action took place)Repeated/habitual actions (e.g. quand jé'tais jeune, je faisais beaucoup du sport)Physical descriptions (this one is easy, it can be clothes, hair, or anything describing someone's appearance).Notice how in all of these cases there is no start and finish to what you are describing. For shorter actions with a definite start and finish, you use the passé composé. Here is an example of a sentence that uses both, to make the difference a little clearer: Lorsque je marchais, j'ai vu un chien - While I was walking, I saw a dog.The best way to fully understand when to use these tenses is to practice writing them as much as possible, and read, read, read.