Anonymous:
A. Can you explain what happened?
B. Can you explain what has happened?

1. What is the difference between the two sentences above?
2. Does sentence B refer to what happened recently?
3. Does sentence B suggest a cause that resulted to the present situation?

eg. The computer is not working. Can you explain what has happened?

4. In #3, I think "what happened" is also possible, though. Would you agree?
5. Which is the more natural verb tense to use for the example in #3?

Please advise. Thank you.
In these specific examples, there isn't really much substantive difference in what you are asking.

What "has happened" puts it more in the recent and potentially still occurring past, where as, "what happened," is more of a definitive past event.

However, in common speech, there wouldn't really be much of a difference in meaning in this example.
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Anonymous:
Thank you very much, sam1947, for your helpful response.

Just a couple of questions more if you could please help me with. Thanks.
Anonymouseg. The computer is not working. Can you explain what has happened?
Does the question "...what has happened?" above expect an answer that is most recent or a still ocurring past, and also in present perfect tense? If so, the answer, for example, could be: The computer has been infected by a virus.

Is it also possible that someone would answer in a definitive past form (and not necessarily in present perfect tense) to the question "...what has happened?"? If so, the answer, for example could be: The monitor displayed a blue screen before it lost power.
The "what has happened" does indicate an occurrence continuing, at least, into the immediate past. The answer could well be, "The computer has been infected by a virus." Or, it could be, "Your computer was infected by a virus." Either could be affecting what is happening now.

The answer to what has happened could also be your 2nd example, "The monitor displayed a blue screen before it lost power." Although, you would probably say, if you were answering the IT person, "I don't know; the monitor displayed a blue screen and then lost power." The latter gives more of a sequence of events through time, which "has happened" seems to call for.

There is just a nuanced difference between the past, "what happened," and the past perfect, "what has happened." The former is more definitively past, although, in the case of computers and many other things in life, that could still have echoing effects on the present.
"Has happened" has more of a continuous feel to it, if not into the present, nearer the present than the definite past.

It is hard to explain, at least for me. One has a sense of these things, but it is hard for me to put it into words for others.

Let's say you know that someone witnessed an accident. Two different time frames would be expressed by asking the onlooker.

What has happened happened? (What you would say if you came upon the scene shortly after it happened.)

What happened? (What you would say if you came upon the scene when there were indicators that it had happend some time in the past--(e.g. officers were there investigating, or there were other signs that it had clearly taken place some period,not necessarily all that long, before you came upon it. It would also be what you would ask the witness the following day. It just doesn't have the same sense of immediate past that "has happened" conveys.)

Maybe someone else can explain it more clearly. It is, afterall, for me the end of a very long and trying work week, so my brain isn't functioning as well as it might at other times!
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Anonymous:
Thank you very much, sam1947, for your detailed explanation. I think I now understand.

If I've understood correctly, "what has happened" describes events that happened just now or in the immediate past, whereas "what happened" describes events that happened in the definitive past like yesterday, last week, etc, although this could also be in the immediate past like 5 minutes ago, I think. However, there is really just nuance difference, as you've explained, that they can almost be used interchangeably. I hope I've got the right idea. If you could, please confirm.
sam1947 The latter gives more of a sequence of events through time, which "has happened" seems to call for.
I see if someone expects a sequence of events as an answer, "what has happened" will be the more appropriate question.
sam1947 It is, afterall, for me the end of a very long and trying work week, so my brain isn't functioning as well as it might at other times!
?

I'm sorry about this, but I really appreciate your time and effort in helping me understand the subject. Again, thank you so much.
I think you are getting it. These really contrast two past time events. If you are just talking about one past time event, you could either use:

Sam had a good year at the track this year. (No contrast to previous years or other time periods is implied by this sentence alone, but it means that the track year has definitely ended).

or, "Sam has had a good year at the track this year." (which implies the year is still going on, or the track year is just finished.)

Sam had had a good year at the track this year. As a stand alone sentence, one would be waiting for something else to follow that....such as, "This enabled him to expand his business." or, "But, his debts from previous years ate up most of his profits."
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Anonymous:
Thanks so much, sam1947, for your additional explanation. It makes sense to me now. Probably my last question:
sam1947or, "Sam has had a good year at the track this year." (which implies the year is still going on, or the track year is just finished.)
With the word "year" in bold above, do you mean the calendar year, which means the game is still going on until it ends on 31 December?

Or do you mean the track year specifically, which means the game still is going on until it ends?
It might or might not be the same calendar year, but would be in or near the end of that track year. The track year might start in Oct. and end in March, say, but people who kept up with the track year would refer to that as "this year," meaning "this track year." For example, pro football season in the U.S. starts in August and continues to the end of January, but people who follow football would consider that one season or one football year, even though it spans parts of 2 calendar years.

But the verb form would mean (with or without the "this year" added at the end of the sentence) that the track year was still going on, although probably toward the end, or just finished, because "has had" is in the past continuing to the present or very near the present.
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Anonymous:
That was even clearer. I guess I have no more questions. Thank you so much, sam1947. I really appreciate it!
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