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 you wish to convey the idea that of the several books you have finished reading, the name of the most recent is Animal Farm:
The last book I read is called Animal Farm.
We say read (past tense) because the statement refers to an event completed in the past; last makes the point that the book was the final book read in a series of books which began and ended in the past. Last does not imply any relevance to the present. In fact, last could be said to exclude the present. Hence the present perfect is inappropriate.
You could also say
The last book I read was Animal Farm.
The last book I read is Animal Farm.
There's no clear case to be made for insisting on a past-tense predicate there.
Also, "read" suggests an act completed. If you wanted to say that the book you have been occupied with, not necessarily finishing it, you could say:
The book I've been reading most recently is Animal Farm.
There, we can use the present perfect.
If you've finished it:
The book I've read most recently is Animal Farm.
"Recently" looks at time from the point of view of the present. Hence, the present perfect is appropriate.
Best Answer
Your simple past understanding is fine. In your example the present perfect and the present perfect continuous are practically interchangeable. In both cases, David is still working for them. With 'how long' questions it is often the case that the two are interchangeable.
Sometimes there is a slight difference between the meaning of the present perfect simple and present perfect continuous. They both have an effect that is valid in the present, but sometimes in the present perfect the action is complete while in the present perfect continuous it doesn't have to be.
A typical grammar book example is 'I have painted the ceiling' versus 'I have been painting the ceiling'. In the first example, the effect is that I am happy now, in the present; the job is done. In the second, the emphasis is on the process, the action. We do not know if the job is finished, just that the speaker was recently involved in painting the ceiling.
PS I should add that this is British English.