How do you know when to use passé simple and when to use passé composé?

Passé composé is a form of past which used to express a past action which affects the present moment. Passé simple is a form of past tense which describes an action which is SIMPLY past, meaning that this action does not affect the present as much.For instance:Je mangeai une pomme avant de partir de chez moi.Je n'ai plus faim car j'ai mangé une pomme.In the first sentence, I am simply describing actions in the past, and explaining that I had an apple before leaving my place. All the actions are in the past and there is no relation with the present when using the verb "to eat".In the second sentence, as I am mentioning that I am not hungry anymore, and this because I had an apple, it implies I just had the apple and the act of "eating" it made me not hungry anymore, so therefore has a relation with the present. So I used passé composé.