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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The perfect tenses (past or present) imply some completed action occurred before some other event and has some relevance to it.
"to go" in general implies motion or a process; thus in the perfect tenses it implies a completed action with continuing relevance. The continuing relevance can in some cases imply the subject has not returned
But not always:
I have gone to England every summer for the past ten years
The modifiers "every summer" and "for the past ten years" rule out the possibility that the subject has not returned (you can't go somewhere you already are).
I tried to change "gone" to "been" in all these sentences and it just doesn't seem right.
Been to actually works in those sentences but gone to seems more natural. Using "been" also seems to shift the meaning slightly in someways.
"had gone to law school" for instance seems to imply completion more than "had been to law school" would.
Is this use really common?
I think your concept of contrasting gone to with been to is too strict. Yes, been to implies somewhere you are no longer (because perfects represent a completed action), but gone to doesn't always mean you are no longer there. It can depending on the context.
Best Answer
The difficulties with your examples arise partly from the use of the verb come as well as with the tenses.
It's clear that you are speaking in Example 1 after you had gone to the party. You are no longer at the party. Therefore you need to say that your friend went to the party. If you were still at the party, your friend would have come to the party.
That's to say, you use come to mean towards me and go to mean away from me or in some other direction.
This is slightly complicated in your examples because you are imagining yourself at the party when you write them.
To avoid this difficulty, you might write:
Example 2 is more complicated. You are writing it before you decide whether to go to the party:
You conclude that:
which is perfectly correct
But because you are not at the party, which you may or may not attend, you need to conclude:
But you cannot say: who came there because you are not there yourself and because this uses the past tense to describe an event that lies in the future for you.