1.
The first question is about the tense we should use after "as soon as". In a sentence such as this:
I had left when the phone rang.
you need to use the past perfect in the second clause to show which action came first and which – second. However, when you use “as soon as”, the sequence is clear and it is normally a matter of preference which one to use, so both your examples will be correct. In American English the preference would normally be past simple. The past perfect would emphasize the fact that one action was complete before the other one occurred. (an explanation given in Grammar for Teachers by Andrea DeCapua)
2.
In the second pair of examples they are both correct again. It is unnecessary to use past perfect because the time is mentioned and the sequence of events is clear. Also, the actions are described in the order in which they occurred. You can use the past perfect if you want, to emphasize that one was before the other.
3.
The third question was about the sentence
He said that the moment he first met her, he felt something special and began to keep a diary.
The actual words the man said must have been:
"The moment I first met her, I felt something special and began to keep a diary."
When you report his words and begin with “He said”, the entire phrase shifts one tense back and becomes:
He said that the moment he had first met her, he had felt something special and had begun to keep a diary.
Although this is the grammatically correct sentence, it is very common that the past simple does not become past perfect in indirect speech. When reporting, native speakers tend to make present tenses past ("I am studying" - "She said she was studying") but very often do not care to make the past tenses perfect, as grammar books always teach us we should.
That is what makes both these sentences correct: "He said that the moment he first met her, he felt something special and began to keep a diary." and “He said that the moment he had first met her, he had felt something special and had begun to keep a diary.” (have a look at the end of this page)
This "authority" is in error; the past perfect is not necessarily required here, although there are circumstances in which it would be preferred in formal discourse, and the simple past is not incorrect, although there are circumstances in which its use would be deprecated in formal discourse.
This is in the first instance a matter of context— specifically, what you are trying to express and what time you are talking about, what grammarians call Reference Time.
Perfect constructions do not express events prior to Reference Time; they express a state grounded in prior events which obtains at Reference Time. So if you are talking about a time in the past when your subject's previous unfamiliarity with bears was of immediate relevance, the past perfect will be appropriate:
As she was hiking in the Tongass National Forest she was suddenly confronted by a menacing dark creature her own height and twice her mass. She did not recognize it—she had never seen a bear before she moved to Alaska.
But if your Reference Time, the time you are speaking about, lies before the time at which she moved to Alaska, a past perfect will not be appropriate:
She was in her youth a keen student of wildlife—kangaroos, wallaby, emus—but she never visited zoos, which depressed her, so she never saw a bear before she moved to Alaska.
In less cut-and-dried circumstances, usage varies: formal usage and British colloquial usage tends to employ the past perfect more often than US colloqual usage. If you have an appetite for technical grammar, I modestly recommend our Canonical Post on Perfect Constructions, particularly sections 3.2 and 4. If not, the best 'rule of thumb' respecting use of the past perfect is FumbleFingers‘ Perfect Truism:
”Don’t use the perfect unless you need it.”
Best Answer
I am an American, and had not noticed this phenomenon. I would have said "I was almost done with my essay when the computer crashed and I found that I had forgotten to save it". So, I decided to check a few N-grams to try to isolate it.
he had decided , he decided (American)
he had decided , he decided (British)
I had forgotten, I forgot (American)
I had forgotten, I forgot (Brithish)
she had lost, she lost (American)
she had lost, she lost (British)
From this data we can see that:
This may depend on the verb in question, but we can expect to read the simple past two to four times as often as the perfect past.
I would assume that spoken English is even more biased towards the simple past than written English.
I would hypothesize that this phenomenon is due at least some of the following factors: