The difference between past simple and present perfect is the latter emphasizes that the action is now complete.
We don’t see in that clip what action “did” stands for, but if it’s one understood to be an inherently non-continuous action, then there is no need to emphasize that it is complete, so past simple is enough.
This form also helps the writers ...
Both are grammatically correct. However, "What are you studying?" is referring to a current, ongoing event, and therefore I would use it to a person who is currently studying, and expect not the major but the particular class, which might not even be in the major.
"What do you study?" is ambiguous. On first hearing, I would not be sure ...
Test verb 1.5 no object, with complement Produce a specified result in a medical test, especially a drugs test or AIDS test.
‘he tested positive for steroids during the race’ ( Lexico )
"I have tested positive" is correct.
"I have been tested positive" sounds awkward and is a much less common usage ( NGrams ).
"I've (or I have) worked there" and "I have been working there" both imply that you are still working there. There is little difference between these so just use whichever you like.
"I worked there" without the 'have' would imply that you used to work there but don't anymore
Side note: it should be "I tend to..." not &...
The infinitive particle is always followed by the base form of the verb, so to meet, never to met.
To met cannot occur in a sentence (unless if the two words are in different constituents and happen to come together, e.g. The lawyer I sent the emails to met up with us last week: The lawyer [I sent the emails to] met up with us last week).
If the infinitive ...
Those are two independent clauses, so there is no difficulty in having two different time perspectives. I think if the clauses were reversed, as in
Only two weeks have passed since the accident, and/yet you left the hospital yesterday.
that would be a more logical expression, but there's nothing wrong with the sentence as you posted it.