We use past simple when the action done in the past, ex: -I went to a new hospital. Here, the action has finished. But in past perfect, the action is in the past but it's effect still continues. Ex: -When they arrived, we already started cooking. (They started cooking before they came and they are still cooking.) -He was very tired because he hadn't slept well. (He didn't slept well in past and he got tired. Action happened in the past but it still continues.)

We use Simple Past if we give past events in the order in which they occurred. However, when we look back from a certain time in the past to tell what had happened before, we use Past Perfect. Simple Past - "Jane got up at seven. She opened her birthday presents and then the whole family went to the zoo." Past Perfect - "Before her sixth birthday, Jane had never been to the zoo."