Una respuesta simple sería: Usar el past simple cuando hablas de una acción en el pasado que no tiene vinculo con el presente. Y especialmente si mencionas un periodo de tiempo que no incluye ei presente. Yesterday, I went to the supermarket. Last Year, I went to Greece. When I was young, I went to five different schools. Usar el present perfect cuando esa acción está vinculado con el presente y, especialmente, si ocurre dentro de un periodo de tiempo que incluye el presente. Today, I have been to the supermarket and now I don't have any money. This year, I've been to Greece and I still have sunburn. I have been to five different schools in my life, which is why I am bad at mathematics. There are other differences, but this is possibly the main one.