You have to think of the order, but it's not that difficult: the "older" thing that happened first is in the past perfect, and the "newer" thing that happened second is in the simple past. Example: "I had fallen asleep before the phone rang."

Hi Ala, it might help you to remember that a past perfect event provides a "historical context" for a past simple event. Sometimes it can even explain why a past simple event happened. These tenses (along with past continuous) all work together. It's much more than the order of events.