Past simple
One event that started and finished in the past.
Past perfect simple
One event that started and finished in the past which links to second event.
You can say:
I went to the supermarket. (past simple)
You cannot say:
I had gone to the supermarket. (past perfect simple)
This is because there is no 'second event'.
You can say:
I had gone to the supermarket before the film started.
This is because there is a 'second event': 'the film started'.
What's the difference here?
1. I went to the shopping centre and ate dinner in a restaurant.
2. I had gone to the shopping centre and ate dinner in a restaurant.
The first clause in sentence 1 is a 'past simple' clause (I went to...) whereas the first clause in sentence 2 is a 'past perfect simple' clause (I had gone to...).
In sentence 1 the two clauses ('I went' and 'ate...') are both past simple and are equal.
In sentence 2 the 'past perfect clause' is dominant. It is the most important. You are saying: I HAD GONE TO THE SHOPPING CENTRE and ate dinner in a restaurant. I imagine the person in this sentence eating dinner in a restaurant near or in the shopping centre.
Conclusion: Both sentences are natural
What's the difference here?
1: I saw my friend last night but we didn't go anywhere special.
2: I had seen my friend last night but we didn't go anywhere special.
The first clause in sentence 1 (I saw...) is 'past simple' and the second clause (didn't go...) is also 'past simple'. Both events are equal in importance.
In sentence 2 we have a 'past perfect simple' clause + a 'past simple' clause. The 'past perfect simple' clause is more important of the two.
Conclusion: Sentence 1 is natural because in Sentence 2 the 'past simple' event (didn't go...) is not a separate event to the 'past perfect simple' event (see). It is a continuation of the same event. To 'see' you friend you have to 'go' somewhere.
What's the difference here?
1: Tom read the interesting new book after he found it at the library.
2: Tom had read the interesting new book after he found it at the library.
We have:
two separate events (find book, read book)
both events started and finished in the past
one event (find book) links to the other event (read book)
In sentence 1 the two past simple clauses are equal in importance.
In sentence 2 the past perfect simple clause is the most important the one the speaker wants you to know about.
Conclusion: Both are natural.
Overall conclusions
Past simple + past simple
Use when two separate events are of equal importance.
Use when the two events are a direct continuation of each other.
Past perfect simple + past simple
Use when there are two separate events that 'link' to each other.
Use when you want to make the 'past perfect simple' event more important.