Without going into too much detail, basically the past simple is used for an action that has finished in a time that has finished. Let's look at a couple of examples.
Yesterday, I played a great game of tennis.
I moved to Indonesia 13 years ago.
Notice the use of the past time markers 'yesterday' and '13 years ago'.
Now let's look at past simple vs past perfect simple, which is the past in the past.
Yesterday, I went for a few beers after I had played a great game of tennis.
Before I decided to move to Indonesia, I had thought about moving to Malaysia.
Here, you can see that the two past perfect verbs happened before the two past simple verbs. Notice also that the past perfect verbs are both 'connected ' to the past simple verbs in some way.
This is a very brief explanation, obviously. If you'd like more, let me know.
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