1. When we give the sequence of past events, we usually put these events in chronological order using past simple form. If we want to refer to an event out of order - that is, an event which happened before the last even in the sequence we have talked about, so we can use the past perfect form.
   1. Ex paragraph. I wrote Clara an email to thank her for the present she had given me for my birthday last week. But as soon as I pressed the 'send' button, I realized that I had made a mistake and sent it to her sister instead.
   2. Ex. Order of events: 1. gave present 2. wrote email 3. made mistake 4. realized mistake
2. When we understand that we are talking about events before another past event, we don't have to continue using the past perfect
   1. Ex. we bought a new car last month. We'd driven my parents' old car for ages, but it started(or had started) to fall apart.
3. If the order of past events is clear from the contest, we can often use either the past perfect or the past simple
   1. Ex. After Ivan had finished(or finished) reading, he put out the light.
   2. Ex. The two leaders agreed to meet, even though earlier talks had failed(or failed) to reach an agreement.