Let's first talk about the following two sentences:
1- Sara went to bed as soon as she had finished homework.
2- Sara went to bed as soon as she finished homework.
I think your confusion is valid because we use the past perfect when we talk about something that took place before another thing in the past. So the use of the past perfect comes across in the first sentence but the use of the past simple in the second sentence doesn't. Am I right? In fact, we don't need to use the past perfect unless it is necessary or unavoidable to do so. Even if we talk about one action happening before the other one, it is possible to use the simple past for both actions if we think it is not necessary to highlight or emphasize the happening of the earlier action. It sounds natural to avoid using the past perfect where the simple past works, which is used to refer to something or several things happening in sequence (one after another) in the past.
So both of the sentences are grammatically correct. However, I'll prefer the second phrase to the first one.
As for the last two sentences, it is correct to say that "everyone had gone home when Sara got to the party", but it's not grammatically correct to say that "everyone had gone home when Sara had got to the party". It doesn't make sense. In the past perfect when we talk about two events, we use the simple past in one clause and the past perfect in the second clause.
Let's now talk about the following sentence you are confused about:
"Everyone went home when Sara had got to the party".
There is nothing wrong with this sentence, but the meaning is other way round. It means that first Sara got to the party and then every one went home. Look at the
first sentence again. When Sara got to the party, everyone had gone home. Here it means that first everyone went home and then Sara got to the party. Sometimes, one action happens soon after the other action, here we should use the past simple in both clauses such as when Sara got to the party, everyone left, when they saw the police, they ran away, etc.
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When talking about things in the past, the past perfect happens before the simple past.
In your example
If someone feels that they had never made a mistake in their life, then it means they had never tried a new thing in their life.
had never made and had never tried are two actions which were started and ended in the past compared with how the person feels (present). The equivalent would be
If someone feels that they never made a mistake in their life, then it means they had never tried a new thing in their life.
where never made is nearer in time than had never tried since one would have had to try before failing.
I had called you
I had gone to the wash room
could individually use the simple past
I called you
I went to the wash room
with similar meaning, however, using past perfect sets up a scenario for something in the more recent past
I had called you before I went to the wash room
I had gone to the wash room before I called you
To answer where you were when they called:
I had gone to the wash room when you called me
means you were in the wash room (past perfect) when the person called
I went to the wash room when you called me
means you heard the phone ring then went (simple past) to the wash room
Best Answer
First past perfect and pluperfect refer to the same tense. They are synonyms. Look up pluperfect in any dictionary to verify this. Here at onelook dictionary are more than two dozen dictionaries to get you started.
Second, please see this answer to When is the past perfect exactly needed? It does a terrific job of stating when the past perfect (or pluperfect--they mean the same thing) is necessary.
In general your sentence 1B is grammatical, but it represents rather an incomplete thought. And it is not necessary to use the past perfect/pluperfect tense here. The present perfect or simple past would both be better choices, because they would not represent incomplete thoughts.
Your 1C adds more information to the response but still either the present perfect or simple past would be more common. You don't need the past perfect/pluperfect there because it is clear from logic which past action happened first, namely that you called him first.
In 2B, there is no reason for using the past perfect/pluperfect tense. The simple past is fine. You mention only one past action. And in fact both uses of the verb order refer to that same past action. It would seem strange to think that one occurrence of the same past action can come before another occurrence of the same!
The first sentence of 2C is grammatical But by itself gives an incomplete thought. There is only one past action mentioned, and it occurs two days back. Thus the simple past is better here because you are talking about one action completed in the past. Having us wait until the second sentence to hear the most recent past action (Yesterday when I opened the box) makes it awkward.
The second sentence of 2C is the only sentence in which the past tense/pluperfect tense is used in a way that is intended. You have two actions and you place one of them before the first one. This is pleasant sounding and appropriate.
For example sentences and a much better explanation, see the answer I link to above. And thanks for asking such a good question.