Learn English – “I askED him if he knowS…” or “knew” – which one is correct

This is an old question, but I want to make a point clear:

Yes, the present perfect is used all the time, by native English speakers, in all registers and dialects of English, from extremely informal to very formal. You cannot sound like a native English speaker without using it when it is called for. (It's not like, say, knowing how to use "whom," which you don't really have to do.)

The good news is that, in a lot of places, the two are interchangeable. Moreover, you are likely to use the past more. Also, using the present perfect in the wrong place will sound very strange and sometimes make you unintelligible, whereas using the past in the wrong place may communicate the wrong shade of meaning but will rarely get your listener completely lost. Therefore, I recommend using the past most of the time, then inserting the perfect gradually if you are sure you are right.

To make sure I wasn't wrong, I turned on the TV now to a sitcom rerun. Ignoring other tenses (mostly present) here is my tally in ten minutes or so:

past 31

present perfect 4

and some other past tense constructions:

"lately, I have been having thoughts" "I think that may have missed the table." "oh God, I shouldn't have said anything" "he kept laughing at..."

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)