With "already", they mean the same thing. The function implied by "already" is "a completed action in the past with present result". The difference between the two types is Americans tend to prefer the simple past version, while everyone else --including Canadians-- prefers the present perfect.
The only exception is with 5. &...
The explanation you found contains the answer. "Since" requires a starting point. "Weeks" identifies a duration, not its start. The following is correct:
Since June he had been waiting for the reply.
This version would be more natural because the starting point is the new information:
He had been waiting for the reply since June.
Just "pandemic started" is correct. "has been ... since the pandemic had started" is awkward and considered incorrect because the perfect tense "it has been ... since" should use the simple past. I like to think of it as "since what specific point in time", and for a specific point in time the simple past makes the ...
Your query about lately is odd, since both your sentences have the same tense, present perfect.
The correct version is
Why hasn't there been any good news lately?
not because it contains the word lately, but because it is a question, and in questions, subject-auxiliary inversion is required.
What may have confused you is the dummy subject there:
"As soon as I had a look..." would imply you're describing something that happened in the past, ie, "As soon as I had a look, I sent them the designs."
Since the action you describe will take place in the future, but is already completed by the time you are sending the designs, that is, you have had a look, you use the present perfect.
If you look at the definition of since in the Cambridge dictionary, it says
from a particular time in the past until a later time, or until now
"from a particular time" must be something like "july" or "2009" or "4pm".
If you want to specify a time interval, like a week or an hour, you have to use for.
for weeks is a ...
In general, the choice of grammatical aspect is, unlike tense, not strictly delineated. You can often choose between two or more aspects to express the same basic idea, because aspects have less to do with denotation, and more to do with connotation. In other words, aspects carry implications and tonality, but they have less effect on the literal meaning of ...
You're completely right. "Have enjoyed" means that the vacation is over (and you enjoyed it while it was happening); "are enjoying" means that you are in the middle of it (and enjoying it right now).
I think people would be more likely to say "I hope that you enjoyed your vacation" rather than "have enjoyed". But with &...
Both options are grammatically correct.
A rough heuristic for which option to choose is shown below. This is not a rule, just a guideline. Thanks to @gotube for the comment about tenses and @stangdon for clarifications.
Option 1 is the present perfect tense. Use option 1 if he's still in the US. "He's spent at least a year in the US thus far, so he ...
He has been having headaches for the past ten days.
This suggests to me that he had a number of separate headaches over the past ten days.
He has had a headache for the past ten days.
I would not use the second sentence as it appears in your question, but I would use it in the form shown above. This would mean that he has had one headache continuously for ...
Present perfect can only be used if the grammar does not indicate that the event has finished.
So, the first "hit" is correct. In this context, it must be a finished time because if there's a situation "after" it, then it must be finished, and therefore not present perfect.
The second one is bad and should be "hit" for the same ...
The first "hit" is fine.
The "has hit" should not be used with a time marker like "before." If a perfect tense were to be used at all, it should be the past perfect, but that is not required because the sequence in time is made clear by "before."
This answer has been edited in response to a comment by Colin Fine.
I don't think there is any great difference in meaning; the continuous tense may give more of a suggestion that the travels, or the visits to the gym, are continuing.
Quite a bit is just another way of saying quite a lot. There is no indication as to how frequent the visits to the gym were.
She has been studying English for two years.
This emphasizes studying as an ongoing or uninterrupted activity up until the present time of speaking. It is also true now unless otherwise specified.
She has studied English for two years. (She is still studying it now.)
The present perfect in this case just tells us she started at some point in the past and ...
The sentence with either tense can be correctly used both when the experience is ongoing, and when it's finished, as they mean the same thing.
There are nuance differences, however. "He has had experience" suggests that the time in question is over, or that it's no longer relevant, or that it's insufficient for whatever purpose we're discussing.
The "has had" construction would mean that while he used to practice those types of law he no longer does so (at least from the litigant's side of the bench). A simple "has" on the other hand would be used where someone is still practicing those skills, i.e. a lawyer's personal advertisements would use "has".