Perfect constructions speak about a state which obtains at Reference Time—the time which your discourse addresses—as a result of some prior event. Present perfect and past perfect address different Reference Times.
Present perfect speaks about a present Reference Time:
I am about to start my first novel, but I have been writing since childhood. ... This speaks about my present writing experience.
Past perfect speaks about a past Reference Time:
In 2003 I started my first novel, but I had been writing since childhood. ... This speaks about my writing experience in 2003.
You may read more (much more, possibly, than you want) about the meaning and use of perfect constructions here.
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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Best Answer
When using did as an "auxiliary" or "helper" verb to form the past tense, it is used with the bare infinitive of the verb in question, the "lexical" verb.
Here we form the past tense using the lexical verb only, which is a shade more formal:
To form the past tense of some verbs, we add the -ed ending to the bare infinitive (mention -> mentioned). With other verbs, the vowel changes (tell -> told).