In French we find many tenses about the past. So when we have to use passé simple and when plus-que-parfait?
This is a common issue for learners of French. A thorough answer would need a book chapter. Here are some suggestions:
Judging from your profile I guess you are Greek. I suggest you look at:
See also the different links inside the suggested web pages. Speaking very briefly you just need be able to recognize passé simple. It is not used orally (at least in colloquial French). Use passé composé instead.
Plus-que-parfait is like past perfect in English or Ypersyntelikos in Greek language. Hence a typical example would be
Quand je suis arrivé(e), il avait déjà mangé.
In this way plus-que-parfait expresses what is called "antériorité dans le passé".
Another typical example:
Si j'avais su, j'y serais allé(e).
which can be translated in English as:
Had I known, I would have gone there.
See below for more examples and exercises: