When should I use imparfait and passé composé?

Passé composé (AVOIR ou ETRE + participe passé) is used for ponctuel actions in the past which have been ended. Ce matin, j’ai mangé une tartine de pain.  On the other hand, imparfait is used for descriptions, background information in a story for example (elle avait les cheveux blonds.). It is also used for actions which used to be a habit or have been repeted many times. (Quand j’étais petite, j’allais au cinema le dimanche). Finally, both tenses can be used in the same sentence. Here, the imparfait will be an action that hasn’t been completed (it is imperfect), which is still going on when the second, punctual action occurs. Le telephone sonnait encore quand je suis rentrée à la maison.