How do I know when to use le passé simple or l'imparfait ?

In French, both le passé simple and l'imparfait are used for past situations. However, they are not used the same way.

Le passé simple is used for short, sudden actions such as :

"Out of the blue, she screamed." --> "De nulpart, elle cria."

L'imparfait, on the contrary, is used to describe situations that have lasted, but are over now. For example :

"She used to scream whenever she would see a spider." --> "Elle avait l'habitude de crier à la vue d'une araignée."